I'm not sure what the cutoff time is for bumping threads, and being twoish months old seems borderline for my purposes, but anyway:
I was able to capture a GCS before it killed any of my dwarves, allowing me to tame it safely. However, I think my dwarves are still treating it as hostile. It is listed as hostile in my unit menu (even though I can set it to be slaughtered from the animal menu), and whenever my dwarves try to release it they prepare a cage beforehand, as though it would attack them at any moment (as opposed to just freeing it like they would a dog or cat on a chain). The spider appears entirely safe, as in my testing I pitted two cats with it and it did not attack them. Even when I pit it and sent a dwarf in for another reason, it neither attacked the dwarf nor caused job canceled: hostile spam.
This has caused a bit of a problem, because in testing out various designs my spider has disappeared from assignment menus; that is, it no longer appears among the list of creatures to assign to a chain, cage or pit. This appeared to happen after I pitted the spider. The doors to the pit are not forbidden, though of course they are pet-impassable.
Theoretically I can just pit it and pit hostiles with it one at a time to have it attack them, but because it runs to melee range it doesn't leave very many webs, and I would rather set up a more automated method.
I'm also having issues with it shooting webs while chained. Whether behind fortifications or not, whether doors are pet-passable/forbidden or not, it does not seem to shoot at enemies unless it is unchained, which has the above issue of not leaving very many webs to collect. Or possibly the issue is simply range? I would test further but I cannot assign the GCS to any new chains.
Any suggestions? Should I not have tamed it?