Now if they would stop being so selective about what they allow on their platform. There is a wide array of popular games available on Steam and DRM-free on Itch but for some reason they aren't available on GOG.

I get that some people have been loudly calling for curation on Steam, but curation is a terrible thing. The customer should be the one to decide what they buy (provided the developer isn't mass-producing asset flips). If they want a middle-ground they can just not promote any unendorsed games.