HereForTheBeer: D/Generation. Just remembered it last week as I was driving from a customer to a hotel. I don't know if it was forgotten, or just plain purt'near unheard-of in the first place.
Not only do I remember that game, but I wrote a sequel to it - rE/Generation (
http://www.dreamcodex.com/regen.php ). That was my first complete game, created using a terrible game development package and rather horrid Poser people. But it was fun to make anyway. I love D/Generation, both on the Amiga and on the PC.
As for forgotten games, a couple I've been dwelling on since finding GOG are Gender Wars (an RTS based on a comedic war of the sexes) and Meat Puppet (a bizarre isometric game somewhat like Crusader: No Remorse). I doubt either are of the kind of quality GOG looks for, but they did exist. (I still have my Gender Wars CD.)
One other title likely forgotten is Commander Blood. An ostensible sequel to the utterly original Captain Blood sci-fi game, it is more of a point-and-click game with muppets. Strange in the way many French titles are, it is charming nonetheless.
Lastly, who remembers Heimdall? Another Amiga release ported to PC, it's one I think could make a useful appearance on GOG, given the right installer and extras.