JakobFel: in my opinion, if you legally purchase a game via one platform, you should be able to play it on other platforms as well... but of course, DRM makes that impossible.
Not really.
When DotEmu was around none of the games purchased from them activated any on GOG, and DotEmu was even a listed publisher on GOG, so there were even some chances of that happening.
So unless you count stores themselves as DRM, then it's not really a DRM issue.
Ironically though, some forms of DRM might let you do that.
If you are buying an old-fashioned software activation key, which obviously is a form of DRM, that should work regardless of where you buy it from, assuming that the actual software is completely the same.
That's how many shareware utility things work even today.