I can't remember who does a more comprehensive survey, but the overall Linux gaming share is ≈4%. So, Steam users are about half of that, if I'm understanding correctly.
No, I mean 4% of the overall Linux desktop share does some amount of gaming. This is based on a survey, and I can't for the life of me remember who does it. It will show up again, with updated totals, on one of these communities eventually.
Some people prefer GOG, some people do itch.io or FOSS games, some people only do emulation. It's possible that I'm misinterpreting the data, too, but Steam having 50% of the gaming share with the other 50% spread amongst other platforms and activities doesn't seem that outlandish to me.