Okay, what’s the biggest and most active gamer community on Matrix?

I don't know, and don't really care. I play games mostly with friends. Listening to a large chat room full of random people doesn't appeal to me at all.

Regardless of social preferences, I think you'll find that there is no Discord alternative with public chat rooms as big and active as Discord's, nor will there be any time soon. The network effect is strong there.

Nevertheless, we can choose tools that serve us better, and invite others to join us when it's practical. Ex-redditors have been doing this with Lemmy. Ex-Windows gamers have been doing this with Linux. Shifting away from an entrenched platform is usually slow and gradual, but not impossible.