You can't really compare old games sales with new games. The reason many didn't buy Fallout on GOG is because it's such a popular game that most already had a working copy and the extras wasn't worth it to buy it again.
Had the old game been hard to get to work on modern systems or hard to find it would have been a different situation.
I agree that it's not a completely fair example but I also don't believe you can in any way definitively say that GOG is outselling Steam on those, because in the end, Steam still has the massive customer base and GOG is still just an up and comer.
Ah and my pointing out of Fallout was not to reason why it might or might not have sold well, it was rather to show that *even* one of the best sold games on GOG has only been sold to only about 10% of it's userbase, which means the bulk of the other games on GOG are sold to an even smaller percentage of it's userbase.
Now granted, overall I wouldn't be shocked that except for a few obvious exceptions like Free games and high profile games a lot of the games on Steam hang around 10% as well (with maybe the more successful ones that aren't free (or Valve) reaching into the 20%'s). It's a bit hard to tell with these numbers.
I'm not saying one way or the other by the way. I don't *know* for a fact if GOG sells more, but I also wouldn't fault a developer for not joining GOG yet based on the assumption that GOG would sell more. Because there's simply no real evidence for that.
I'm not going to argue sales figures here because I don't know and doesn't want to guess.
My point is that comparing GOG to Steam isn't as easy as some might think. Different sized customer base and also overall a different type of customers.