vsr: Now I wonder: Why GOG's client can't act like Steam or Origin client?
Well, you said yourself Origin doesn't support XP anymore, so...
Steam, on the other hand, was developed originally in the XP/2000 era, which explains better why it still might have support for XP.
vsr: Steam client works on Windows XP and doesn't cripple games, which were developed for Windows XP. Steam client still receives official support and updates on Windows XP.
I am quite sure they will drop XP support in the Steam client at some point. I was there when Steam suddenly dropped support for Windows 2000 (my PC at the time was running Windows 2000, and I had Steam on it), the client simply refused to work on 2000 and gave a note that it doesn't support Windows 2000 anymore.
XP support in Steam may be prolonged due to ten billion Chinese still using it, apparently. But when those finally migrate to something newer, *poof*.
vsr: Glad to hear that! Probably publisher haven't tested it on Windows 7/8/10. Or there are reasons that we are not aware of.
I've also been running Gothic (GOG) on two different Windows 7 PCs fine.
vsr: Nonetheless my question applies to any title, not only Gothic 1. Pretty sure there are games on GOG, which work only on XP/Vista.
I can understand there is a difference between XP and Vista/7 (e.g. Aero in Vista/7 caused the "rainbow color" glitch in many games, didn't it?)... but I can't understand why some game would run fine on Vista but not on 7? Aren't those two Windows version quite much the same, a bit like Windows98 and Windows98SE? Ie. if there are some compatibility issues in Windows 7, then they will probably happen also on Vista?
Or do you have some examples of games which work just fine in Vista, but if you run the on Windows 7 on that same PC, problems? Let's presume both are using the same "bit-version" of Windows, ie. either the 32 or 64 bit edition.