The documentation related to the Google Play Services Sign in HERE is really outdated since Google changed the Developer console and the Play services and Authenetication structure.
I tried the Sign in example project, the playfab version there is pretty outdated too. Anyway I was unable to get the stuff working, and guessing the outdated instructions of the doc is quite hard. I was able to activate google play and init the login sequence, which prompts the login popup and the permission asking, but after accepting I just get an "Authentication failed" error, which I think is probably some incorrect or missing step in the Google configuration.
I searched everywhere about the documentation above but cannot find an updated tutorial or docs.
I used Playfab in the past and was able to get a solid and stable framework and I think you did a great work, but I'm a bit surprised about the outdated Authentication docs.