There have been a couple of wrappers written for Google Play Games Services - originally by NJ Dude, then more recently by Informatix (his library can be found
here). I don't know if you can implement GPGS in B4A using direct Java calls - I think it's probably way too complex.
I have used NJ Dude's library for Achievements & Leaderboards & find it works well - even though it uses v14 of the GPGS API. The problem with it is that it doesn't work for multiplayer games over Google Play.
Informatix's library does have an implementation of multiplayer (although I think only turn-based (I could be wrong)), plus Achievements & Leaderboards, but I was never able to get it working successfully in any of my projects, so I ended up just using NJ Dude's one & gave up trying to put multiplayer in any games - except for a recently released a Tic-Tac-Toe game developed with Android Studio & featuring real-time multiplayer using GPGS - but that doesn't really help you.
I'd say start with the link to Informatix's library & also search the forum for NJ Dude's & see where that takes you.
- Colin.