RetroJaro: The trouble companies I can see are blizzard, maxis, id software, apogee, and epic megagames. Plus any games that were originally on a console (NES, SNES, Genesis, etc.). I would like to see macintosh games on PC as well, such as the mac version of wolfenstein 3D, and firefall arcade for the PC, but I doubt this will happen.
Still, the release of the old duke nukem games gives me hope for apogee, but from what I understand, doesn't gearbox have the duke license now, and that's why it was able to be released?
Gearbox bought the Duke Nukem IP from 3D Realms(a company that very much still exists) except 3D Realms retained the rights to all games released before(and AFAIK in development as well) before DNF. In 2007 3D Realms(Which to this very day is legally called Apogee Software, Ltd) licensed the Apogee name, logo and some of the back catalog IP's to a new company set up by a couple of entrepreneurs.
But both companies are signed up to GOG, so any back catalog game published by Apogee/3D Realms where the original developer don't have exclusive rights to may well come to GOG. In fact the new Apogee the Rise Of The Triad remake by Interceptor Entertainment and that is supposed to come to GOG, also when 3D Realms recently signed with GOG with the release of Shadow Warrior(Which by the way just released for iOS with help of developer General Arcade) and Terminal Reality, GOG said more games were coming. Since then we have(like you said) seen the release of Duke 1 & 2 and may well see more games. So not impossible or unlikely at all.