easy FB farm ^^ cap GCS, do this by leading it down a trapped hallway (gcs are not trap immune are they? didn't see it in its raws) after capturing said spider, put the cage in the center of a Large room with paths leading up from the caverns and also from the outside world (to capture titans and other unfortunate beings) link spiders cage to lever somewhere safe, and proceed to wall the spider in on all sides with fortification (gcs webs are a ranged attack and should pass through fortifications no?) put doors/flood gates/ something else that can not be destroyed by building destroyers while open around outside of fortifications, links said objects to lever.
Trap entire room with cage traps. release spider. Open up fail safe drawbridges (these would need to be installed prior to gcs installation/room access to caverns/outside world) possibly with a fail safe pressure switch to another drawbridge leading into your fortress, just incase something slips by.
open all drawbridges and the doors around the fortifications (to allow spider to shoot webs and stop FB from destroying them), watch FB and other fun things come in and get webbed and trapped. close draw bridges, close doors around GCS, go in collect FBs and spare webs, do all over again.
this is a bit complicated and may deserve some more dwarven science put into it, as i'm relativly new to this game (i've only ever breached the first cavern layer, and that was just today, i have yet to stand off with either a FB or GCS, or anything else that fun, as a matter of fact my only brush with anything hostile was a goblin ambush that killed my 3rd fortress, the is fortress number 5.)
And of course it has lots and lots of implications as far as fun go, mis build the drawbridges/ doors/anything. and your dwares will get webbed, your fortress destroyed, and who knows what else, but this sounds like a much more fun way to get rid of fbs then standard lava/collapse/water trap