mk47at: Not universally true. Search for the article about the missing Linux version of the Metro games. GOG have admitted that they refused the Linux version because of unspecified technical issues.
... but this doesn't disprove my point. :o Even in that case, GOG is not responsible for the game having technical issues, nor should it be their place to fix them. Again it rests on the devs.
The cases where GOG takes it on themselves to resolve technical issues is from a storefront standpoint - they have a service to their customers to uphold, to fix technical issues that come from GOG's installers, or to liase with the devs to fix in-game problems. I imagine this is a whole other story with old games where the dev company is defunct... it seems GOG takes it on themselves as much as possible, at that point.
I don't know the full story with Metro, since I don't work for either company. But I know there are a few games that have been pulled due to technical issues that the devs refused to fix, so GOG no longer wanted to sell the games. Such actions being a good or bad thing depends on your stance: would you rather have a mess, or not at all?
(Though there are still many games on the store that you could argue should be pulled for the same reason... I have no idea what GOG's mentality is about this stuff, since it flipflops all the time, lol)