Klumpen0815: Why would anyone want to use Steam when it doesn't even has those gems?
Leroux: Obviously, because you could also list a lot of great games on Steam not available on GOG and the choice from which store you buy doesn't have to be an exclusive either/or choice - you can get the best of both worlds. The DRM is a much more understable reason not to buy from Steam than Steam not offering some of these good old games.
The problem is that Valve has used some pretty unsavoury lock-in tactics through its Steamworks API that discourage developers from releasing on other platforms. If well-designed in a low-level development environment (i.e. coded in C or C++), games can work just as well outside of Steam as with it, but the API is designed to reflect the fact that developers will frequently take the path of least resistance, which means that elegantly handling the absence of the Steam service involves extra work, and Valve does not actively enforce this.
A number of Steamworks implementations in widely used middleware solutions for example require Steam to be running when the engine initialises, and for developers using higher-level solutions like GameMaker, RPG Maker or Unity, ensuring that the game doesn't simply exit with an error message lamenting the lack of Steam service can be problematic when the middleware itself causes the game to exit with an error message.
GOG lends itself particularly well to older games because these are usually hacked binary releases which do not allow the Steamworks API to be implemented.
The Galaxy API, as far as I can tell, has the same problems, but I expect that GOG's QA requires that Galaxy API users implement a failsafe that enables games to run outside of the Galaxy client, similarly to the way that Sony, Microsoft and NIntendo's QA procedures require that (fundamentally offline) games on their platforms can be run offline without the game throwing up an error.