It seems I'm the resident GCS expert, at least looking at these responses... Being that I've refined my trap-webbing technique and I've caught nine (9) FB/Titans in my current fort, yeah...
1) Wild GCS will shoot at anything they can path to. Tame GCS will only shoot in self-defense. Exposing your GCS to danger is clearly bad, so it's preferred to leave them wild so that you can utilize pathfinding.
2) Wild GCS will only shoot at things they can path to, even if the actual shot is over un-pathable terrain. Namely, if they have a row of fortifications, and there's an opening at the end of the row, they'll shoot through the forts as they run to the opening.
3) GCS can destroy wooden objects, like wooden doors and hatch covers, but cannot destroy stone.
4) GCS will trigger pressure plates, and hatches activate instantly.
All this in mind, it culminates to make my very fun cage-webbing setup, which is...
Those two webs in the center are stray webs and have nothing under them, so consider them "empty space". The GCS is placed on the lower side, then the door is locked, and it's released via lever. It runs towards the chained animal, which is not pictured here, and hits one pressure plate. Then it notices the other route, and runs the other way, hitting the other pressure plate. It does this repeatedly, and very often shoots web while running. I've set this up so that I can open the door and let the GCS run down and cage itself while trying to escape, and that 1x1 bridge at the top can be raised to stop Urist McSuicidal from getting himself caged. The stairs you see offer an alternate route, which is just a short hallway with a drawbridge and some cage traps. This is so that Urist McSuicidal can go over the whole thing and avoid the spider already in progress, and the cages collect various trolls/crundle/more-GCS that try to get into my fort. When a FB appears, I simply close the upper gate, open the lower gate, and the FB runs in to kill my bait animal, getting itself caged before it can actually kill it. Peacocks are my preferred bait, as they don't eat and they lay eggs like wildfire.
As per the original post, this could be modified to leave the GCS a path -under- the cage alley, with fortifications to allow web movement, and a hatch to prevent the spider from actually leaving the enclosure. This should, in theory, make the spider path towards the bait on the other side, webbing as it runs, then stop when its hatch disappears, but I've not tried a mid-placement web target, as I prefer the back-placement web target.
Still, it's much much easier to web the cages, block them off, and then unblock them in event of [TRAP_AVOID] because having pre-webbed traps in the open, or trying to web traps on demand, results in lots of Fun. Also, archers will get your GCS if it's just got forts protecting it, and that's not cool.