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Android Studio Gradle dependency doesn't show up in External Libraries

I'm using Android Studio 3.0.1 and I'm trying to add an online dependency and while Gradle initially syncs without a problem it doesn't show my dependency in External Libraries and my code that references the dependency doesn't work.
Here's a snippet of what my build.gradle file looks like:
repositories {
mavenCentral()
maven { url 'https://oss.sonatype.org/content/groups/public/' }
}
dependencies {
compile group: 'com.fortysevendeg.android', name: 'swipelistview', version: '1.0-SNAPSHOT'
}
I'm pretty new to android development (took over an existing project from a dev who quit without leaving any documentation) so I'm not sure if this is a mistake with how to add a project dependency or if there is a problem with the dependency that I'm trying to add. Any help would be greatly appreciated!

I was able to get this to work by changing the dependency declaration to:
compile group: 'com.fortysevendeg.android', name: 'swipelistview', version: '1.0-SNAPSHOT', classifier: 'jar-with-dependencies'
The library artifacts up on the repository include an apklib and a JAR with a special classifier. The apklib format is not supported by Android Studio, and unfortunately the classifier on the JAR means that it's not accessible simply using the group-name-version format when declaring dependencies.

Your build.gradle file seems fine. If you want to keep the library specified as an external library, you can try and define the dependency using the alternative notation, replace:
compile group: 'com.fortysevendeg.android', name: 'swipelistview', version: '1.0-SNAPSHOT'
with:
compile 'com.fortysevendeg.android:swipelistview:1.0-SNAPSHOT'
The alternative approach is to download the jar file yourself and use it as a local dependency. If you navigate to the maven repository you can inspect the package which is included as a dependency and download the jar directly. Place the jar file in the libs folder of your project and add the following to your build.gradle file:
compile fileTree(dir: 'libs', include: ['*.jar'])
For further details on how to configure the dependencies of your gradle project, check out the Android Studio documentation here.
Based on the information you have provided, this should fix your issues. If this does not solve the error then there may be other issues with the project.

Your dependencies should not placed in the top-level build.gradle file where the repositories are defined. There is even a comment in that file that says so, by default.
You app dependencies should be the module's build.gradle along with the others like android-support
Additionally, that library is very old, and is a SNAPSHOT build, meaning it isn't meant to be generally used in a release environment. You should find an alternative... And there are plenty of other ListView swiping ones

Include /libs/ folder in aar

Feel like Im going mad here - this must be so simple!
I have an android aar which I have built from gradle assembleRelease and also using the maven-publish plugin. I thought that /libs/ was included by default but evidently not.
Android tools site shows its an optional include
http://tools.android.com/tech-docs/new-build-system/aar-format
but for the life of me I don't see where this is configured.
I have asked a related Q Include folder in Gradle artifact but I dont see this as a duplicate as thats a generic gradle question really whereas this is aar specific and may be solved outside of gradle.
Edit I have also asked on the Gradle forum

The aar packages local libraries in libs/ so you need to have local jar dependencies.
dependencies {
compile fileTree(include: ['*.jar'], dir: 'libs')
}

When the maven plugin runs uploadArchives then it will create a pom file that tells maven or gradle what dependencies your aar needs. Gradle will handle downloading the jar files and placing them in your class path for the build.
I commented on your other question as well
*reference: https://maven.apache.org/pom.html#Dependencies
*=gradle is backed by maven for dependency management so artifacts available to maven are also available to gradle and vice versa

How is "libs" created and maintained in a gradle android project?

I currently am not using the "libs" folder for my third party dependencies (it seems they are added automatically to build/intermediates/pre-dexed/) but noticed that it may help static code analysis so I would like to add it to the project.
Note: I'm using maven dependencies.
My question: Are people using custom scripts to generate this folder? I hardly think that this is generated once and then manually maintained when there is a newer version available.
Please enlighten me!

With Android Studio AND Gradle, there is no need to use libs folder (except for old .jar library).
In fact you can develop Android app whitout Android Studio as in your build.gradle there is already a apply plugin: 'com.android.application'
Gralde is using Maven or jCenter via gradle dependencies to import libraries. Gradle and Android Gradle plugin will automaticly download the libs as you sayed in a build/ folder. It is not static and can be clean with the Clean projet on Android Studio. Also, Android Studio will add a warning when a new library version is available automaticly in your build.gradle.
Dont miss the old libs folder used to import .jar library

I currently am not using the "libs" folder for my third party dependencies (it seems they are added automatically to build/intermediates/pre-dexed/) but noticed that it may help static code analysis so I would like to add it to the project. Note: I'm using maven dependencies.
Don't confuse the libs folder with build/intermediates/pre-dexed/ folder.
Currently the gradle plugin for Android manages the build process and create these "internal" and intermediates folders.
My question: Are people using custom scripts to generate this folder?
I hardly think that this is generated once
You don't have to create this folder. The Gradle plugin for Android manage it for your. Also it will be deleted and recreated when your run a gradlew clean command.
and then manually maintained when there is a newer version available.
No. Your dependencies are defined in your build.gradle file.
When you define a new version, Gradle downloads the new dependency and updates the intermediate folders.
You can define your dependencies in many ways:
dependencies{
//You can create a folder and put jar files inside. You can use your favorite name, usually it is libs.
compile fileTree(dir: 'libs', include: ['*.jar'])
//The support libraries dependencies are in a local maven managed by SDK
compile 'com.android.support:appcompat-v7:23.0.0'
// A Maven dependency
compile 'com.squareup.picasso:picasso:2.5.2'
//A local library
compile project(':mylibrary')
//An aar file. It requires to define a repository.
//repositories{
// flatDir{
// dirs 'libs'
// }
//}
compile(name:'nameOfYourAARFileWithoutExtension', ext:'aar')
}

Import library in Eclipse without jar

I'm using Eclipse for developing an Android application. I need to use a library that can be imported using gradle (from Maven repository), since the jar is not provided. How can I do this?

Have you try this link?
Create a new folder called basic-eclipse.
Inside the folder, create folders src/main/java.
Create the Gradle build file in the basic-eclipse folder with the following contents:
apply plugin: 'java'
apply plugin: 'eclipse'
repositories {
mavenCentral()
}
targetCompatibility = 1.6
dependencies {
compile 'commons-lang:commons-lang:20030203.000129'
testCompile 'junit:junit:4.+'
}
From the command line, run gradle eclipse
Now, go into Eclipse and choose File -> Import -> Existing Projects into Workspace and choose the basic-eclipse folder that you created.
import-eclipse-project Notice that src/main/java is automatically configured to be the source directory. Also, the Commons-Lang and Junit jars have been downloaded and added to our build path automatically (in addition to Hamcrest, a jar that Junit depends on).

Unfortunately, the Android Development Toolkit (ADT) does not yet support AAR dependencies.
Please see here for more information:
https://code.google.com/p/android/issues/detail?id=59183
There is an open ticket for the Android for Maven Eclipse project to bring AAR support to Eclipse:
https://github.com/rgladwell/m2e-android/issues/177
Also, the Andmore project is looking to bring AAR support to Eclipse:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=459569

According to your questions,you want to import '.jar' and '.aar',I think you should chose AS.

The best way to integrate third party library in Android studio

We can find some very good open source libraries for android. I want to know what is the best way to integrate them to our own projects in Android studio. Here are some basic methods:
Copy the source code and resource files into our own project. We need to change a lot of codes (the package name, and the name in xml,etc)
If jar files is provided, I just create libs folder for my project and copy the jar files inside. And add the jar file in Module setting's dependencies. But unfortunately I got a lot of error messages like "Gradle: Package com.google.gson doesn't exist".
Is there a general rule to add third party source or jar files into an existing android studio project? Thanks

I prefer to use central repository for dependencies management. So for gson 2.3 dependency you should add to build.gradle file:
Specify that you want to use maven central repository for your dependency
repositories {jcenter()}
Add compile dependency to gson 2.6.2
dependencies {compile 'com.google.code.gson:gson:2.6.2'}
Android Studio as well as your CI server should easily build your project now. And you can continue app development.
I prefer to use central repository for dependencies management because:
easier scope management - some libraries are only for testing, some should be included to apk and some are part of running environment (like android.jar itself)
easier transitive dependencies management - it is quite hard to collect libraries dependencies and if you use "jar-with-dependencies" you could get error "class already added" during dexing
lighter repository and easier dependency upgrade
Examples:
Robolectric jar should be used for unit testing only and shouldn't be part of apk itself
Repository is clean from different folders with jars, checkout takes much less. No needs to download and replace old jars with new jars
I should notice:
Not many libraries are in maven central and you should make some effort to use them such way in your project
You could much easier get to "class already added" error during dexing with central repository approach
You can mix usage of dependencies from central repository and from lib folder, but I prefer to use only one way for simplicity

Put the Gson jar (in my case, gson-2.2.4.jar) into the libs folder
Right click it and hit 'Add as library'
Ensure that compile files('libs/gson-2.2.4.jar') is in your build.gradle file
Do a clean build (you can probably do this fine in Android Studio, but to make sure I navigated in a terminal to the root folder of my app and typed gradlew clean. I'm on Mac OS X, the command might be different on your system
This series of steps was taken from Android Studio: Add jar as library? and is not my original answer. I am posting them here, again, because your question was the third in search results on Google when looking up this same topic. Hence, copying.
All credits to the one who wrote the steps.

Download & Copy Your .jar file in libs folder then adding one line to build.gradle:
dependencies {
compile fileTree(dir: 'libs', include: ['*.jar']) ----> AS creates this
compile 'com.google.code.gson:gson:2.3.4' ----------> I added this one
}
Do not forget to click "Sync now"
I´m using Android Studio 1.1.0

Download and copy your jar to libs folder then add the following to your app.gradle file and SYNC.
dependencies {
compile 'com.google.code.gson:gson:{version_you_need}'
}
repositories{
flatDir{
dirs 'libs'
}
}