Sorry about the confusion about the documentation, we’ll improve it to make it more clean and clear. GPG requires a lot of documentation and it’s on going.
Do you just want to use leaderboard and achievements? or you want to implement things like cloud save and multiplayer as well?
If you just want to use leaderboard and achievements, I would recommend using sdkboxplay since it will be way easier.
If you want to use the features like cloud save, and multiplayer then you should use gpg.
Regarding how to get google play service. check out this sample