Wishbone: That's the case for new games I'm sure, but I imagine that for old ones that's less of an issue. It's probably more a case of "is it completeable on this setup?" Remember, GOG doesn't develop the games. The "obscure bug trigger testing" should hopefully have been done by the developer.
This is true, but you never know what kind of behaviour can crash a game on Windows 7 for example. Right mouse button not working on some specs, on some specs and windows versions if you double click somewhere the screen goes black etc etc.
GOG testing may be not that through and through like with new games, but I believe Carmageddon 1 wasn't being tested for few months just so GOG testers could play it a little longer :P
"Is the game completeable" test clearly isn't enough when a game can be completed in a different ways, choosing different dialogue options, etc etc.
I'm sure GOG wouldn't release a game on a "well, you can complete the game, but you can't take NPC X with you, you can't use sword Y and you can't use the spell Z because it crashes the game on Windows 7" basis.
I know they can't bugfix a game because they don't have a source code, but if the game is completely unplayable on operating systems more modern than windows XP, gog wouldn't release such game.