You're presuming the contributors to Lemmy are just the same in their choices of gaming as the broader market.

It doesn't take that much reading of posts in Lemmy to conclude that it's heavilly biased towards adults, techies and lefties.

In Statistics you can only make presumptions about a subset of subjects from statistical distribution data from the whole universe of subjects if the subset has been randomly selected, which this one most definitelly hasn't - if only because of the "Reddit migration" Lemmy is filled with people with a certain kind of mindset (the ones for whom the actions of the Reddit CEO were displeasing enough to make them want to move and who actually had the will to do so) which isn't at all the average person's behaviour (the "average" just stayed there) plus even the Reddit population was already not representative of gamers generally (older in general).

The general market share of GOG might give you a hint that here too it's likely going to have fewer customers than something like Steam, but judging by comments I've read here it's probably more than 2%, at least amongst commenters (no idea about lurkers).