I would say, with regards to a nerf, that this is much more a question of what you want the mod to be about than the merits of GFS as an individual opponent race. What I love about Fortress Defense (FD) is the way the game is constantly throwing new problems at me. What I hate about FD is how some of those problems are beyond the ability of my current play style to solve. As a long time DF player I'm used to goblins... which become something of a joke once you have a capable military. In FD (for my current fort where not all races are available) this is true of some races (Raptor men, Furies, Blendacs) but not others (Hellfire Imps, GFS, Ruthererturs).
I like the idea of having enemies visit who will just wreck my shit if I fight them like goblins. That adds a new dimension to the game that is badly lacking in vanilla. Other races, imps and rutherertaurs, present serious challenges but they can be taken care of through careful planning and lots of preparation. Imps are just stupid dangerous due to fire. At least with the civ in my current world the rutherertaurs show up in sieges of 100+, which is enough to overwhelm almost any number of dwarves. Extensive testing has shown that in an open fight those kind of numbers coupled with the other things rutherertaurs having going will just keep trucking until every military dwarf is dead. This is cool and fun because it gives me a chance to break them into smaller groups and take them apart, like how with imps crossbows and whatnot become important.
The problem with GFS is not so much that they are very dangerous, but rather that there is not really a way around that. Webs make them brutal, and no matter how you stage the battle this won't change. This isn't bad really just a bit frustrating at times. The player has the choice of either locking the doors and waiting them out, or dying. A third option, one I haven't set up yet, is the "wall of traps that kills anything ever" which is a highly effective defense but because of how the game works also a bit overpowered. A hall of doors that lets me break them into groups of one or two does basically the same thing as five hundred steel spikes hooked to a lever; it simply crushes the problem with engineering since GFS are helpless against doors.
So the question to you as the mod developer is what you want out of your hostile races. I take losses from fighting lesser attackers on the open ground. A hundred furies versus twenty dwarves means one fury is going to get lucky and I as the player have to deal with a loss or two. A hundred Rutherertaurs means big problems, missed migrants and caravans, and god help me if I don't get everything set up fast enough. GFS mean lock the doors or savescum. I'm working on a trap setup that will end them, but is forcing the player to rely on predictable AI pathing and the full power of traps a design goal? If so, they are working. If not, is there a way to make them more unpredictable/interesting?