I am integrating OAuth login for Google+ on my android application, following the tutorial.
According to the tutorial, I should add the Google Service plugin by addi
I would advise you against doing what you are doing. I did it too (by following Set up a GCM Client App on Android blindly) but ended up with exceeding 65K method limit.
So, you should remove the following lines from your gradle:
apply plugin: 'com.google.gms.google-services'
classpath 'com.google.gms:google-services:1.0'
Now import APIs of Google Plus with simple gradle import:
compile 'com.google.android.gms:play-services-plus:8.1.0'