AB2012: GOG actually has 3544 games. And no I wouldn't switch to Steam because a DRM-Free purchase filter is not a DRM-Free distribution guarantee, ie, it does nothing to address the two issues of
1. No offline installers
That is annoying, but it's only a one-time problem.
When you use Steam client to download the game once, after that you can just back it up and use it later as you like. In some ways it's even more convenient than GOG installers. For instance, with GOG games I have to back them up twice, first as installers and then as installed folders to have them readily available.
Obviously I prefer GOG for a number of reasons, but I can't see the lack of installers as such being a big problem, if games are otherwise DRM-free.
AB2012: The Steam version of Cognition: An Erica Reed Thriller can be "played" but literally can't save the game without the client running (which in a 19hr long game means it's practically not DRM-Free)...
There are some games like that unfortunately.
I believe in most cases that is really some unintentional side effect of achievements, which is one more reason to hate achievements.
Because they are selling the game DRM-free elsewhere, it makes no sense for that to be intended DRM. I have seen some games which have integrated Steam client more deeply than they actually would need to, with the game being broken without the client.
I assume that it is unintended from the developers' side, but then again achievements are part of Steam DRM as written in Steam documentation, so perhaps from Steam's side it is working as intended.