I’m having the same problem. I had already configured the OAuth client to use Google Play Games services. It was working correctly, but yesterday my testers started reporting that they couldn’t log in to see the leaderboard. When I went into the Google console, I saw a message saying that I had to configure the OAuth consent screen before adding credentials (I already had the credentials). Now if I try to go to the “OAuth consent screen,” I’m redirected to the “OAuth overview” page.
I don’t know what to do anymore… I need help.