LordKuruku: In other words, I would recommend being
very patient. GOG, as well as others, no doubt Steam, are well aware that people want it. In fact, the first one to get it will probably make a hefty sum in the first day.
Honestly I don't think so... no matter how popular System Shock 1 or 2 might be, their "sales potential" is, in my opinion, most probably ridiculous compared to a Portal 2, a CoD, or any other recent AAA games not to mention that the game will be a lot more hassle to support than a modern game.
Also, unlike GoG, Steam only sell games they don't tweak them to work on modern system, so they probably have no interest in selling them, at least not until somebody else manage to make them work on modern computers.
EDIT: Personally I am pretty sure that there are plenty games that aren't sold on GoG not because of licensing issues or anything but simply because it's not yet possible to have them to run reliably on recent OS.