I agree about it being an optional native extension and not part of the core.
The thing about GPGS and any platform specific services like these is that they seem to make whatever platform provider want to promote your apps more since you actually use the services they provide. That reason alone shows the importance of easily supporting these features.
Most compelling features of GPGS to me:
- Achievements
- Leaderboards
- Saved Games
Once Defold gets support maybe it could be added here too https://developers.google.com/games/services/integration/