Most like GG are Blazblue and Melty Blood. Melty blood is a natural fit for GOG as PC friendly versions already exist. SF2 will be a tough sell given the amount of emulated versions. a SF2/3/Alpha collection might fair better, but truthfully signing Capcom won't be easy because they do so very little for PC, even if they wanted to join the roster it would cost them money to make Vista/Win7 compatible versions of their games that aren't just straight up ROMs running on a slapped together cruddy emulator. Lord knows they won't package their games with MAME, as hilarious as that would be. Other than that there is SF4, but that's frankly too new to be a GOG release anyways, it's still selling for big money considering all the expansions.
Other fighting games available to PC are few and far between. GG is truly amongst the best. You have DOA online which went nowhere fast (and is still too new for GOG), fxFighter, which is the right age, but isn't what I'd call good, and well... Ultimate Mortal Kombat 3.. which I hope has appeared on GOG's radar. A hard sell, like SF2, even at 6 dollars, but I couldn't agree more, GOG needs a stronger fighting game scene, especially ones that can link players here in the forums.
It's funny.. PC gamers often blast console gamers for what the FPS genre has become, citing the lack of difficulty and nuance.. But at the core of competitive and nuanced gaming is the fighting game, as pure as it gets.. And they are nigh on completely absent from the PC gamers realm.