Are you using C++ or Blueprints?
I will explain it to you:
In your config you have to specify GooglePlay as your OnlineSubsystem. To look how to do this look in the OnlineSubsystem documentaion.
First uncomment the OnlineSubsystem part in your BuildScript.
You can obtain an OnlineSubsystem pointer by
From there you can get different Pointers for example to Achievement Interface or Leaderboard Interface.
For the functions look github/UnrealEngine.
GooglePlay implements the following interfaces properly:
ExternalUI (to display the info).
Look the Tappy Chicken example project to see which nodes are there and how they are used, it is a good example!