How to integrate Firebase to Libgdx?

I have an android game built with libgdx and what I’d like to do is to add some kind of authentication so I can connect the game with user data to firebase database.

I have some experience with firebase using it in native android apps, but I have no idea on how to connect "native code" with libgdx.

Researching I found these third party libs and

But as it does not provide any example it didn’t help me much.

So what’s the best way to "mix" native android code and libgdx? I want to be able to achieve things like storing data on db, implementing in-app-purchases, authenticating users. If I’m looking the wrong way please let me know too.