cogadh: GOG creates its own installers for the games and modifies the games as necessary to work on modern hardware and OSes. As for a guarantee, no PC game is ever actually guaranteed to work, regardless of its age or where you get it from, but with very few exceptions, all of GOG's games work exactly as they did when they were new.
EDIT - Also, you might want to check out GOG's
About Us page for a bit more information on what they actually do and the sticky topics at the top of the forum. Oh, and I almost forgot: welcome to GOG.
Thanks a lot for the replies guys (and the welcome).
I figured they'd be doing stuff like that, and I can see that a guarantee would be iffy when you're dealing with various rigs and setups and undoubtedly a million human errors, but I just wanted to make sure that they did a lot to ensure that most people could play these games.
I'm gonna roll the dice, the prices aren't exactly gonna ruin you here.