Forget GoG, its nothing like Steam...
I already own a copy of Freespace 2 in a Box, that i brought back in 1999... ill more than happily buy FS2 again if it were on steam...
Just imagine FS2 on steam, with FSO as a free community expancion pack, with steam archivements for medals and some other crazy stuff that it could be added... it will requiere a lot of work(mostly on pilot stats sid, and the whole game needs to be intregrated with steam), but it will be impressive.
I dont see why not, even if the full game were free, they will be happy to have a new free game, its a good way to catch customers, personally i first installed steam to play a free game called "Alien Swarm" with some friends, now i own another 5 games on steam...