Android Studio - GoogleAuthUtil cannot be resolved

You need to add play services auth to your build.gradle's dependencies like this: com.google.android.gms:play-services-auth:10.2.0

See more here: https://developers.google.com/android/guides/setup

This is a dup of a few other questions however:

Assure you have followed the documentation, make sure you have installed the Extras/Google Repository from the SDK Manager and added the following dependency to your <project>/<modulename>/build.gradle file (not the root .gradle file):

dependencies {
    compile 'com.google.android.gms:play-services-auth:16.0.+'
 }

Next sync your project and Gradle files using then rebuild your project.