Bit harder. It needs a bait animal at most 4 tiles away from the spider, and you need to fool the spider into thinking it is able to path to it, by using a locked, but pet-passable door in the spider pen. Since you don't want the bait animal to trigger the webbed over traps, you need to chain it. This creates 3x3 blocks of non-trappable space. It's a bit of a puzzle to make an effective setup. Raising bridges of 1 tile lenght are one solution
W..S.W W - wall F - fortification
WdFFFW . - floor S - GCS
..^^^. d - door, locked but pet-passable
..^^^. ^ - traps you want webbed
====== ===== - bridge, raises to north or south, raise it once the traps are webbed to leave a 2 wide trapped corridor and make the spider stop shooting
...v.. v -chain with bait animal
WWWWWW
I think that *should* work, although it's been a few versions since I toyed with GCS.
Now when you capture something in a webbed trap, you want to forbid the cage until your weavers have cleared the webs away, unless you want to cage your own dwarves.
Note that when facing building destroyers, you want the pet impassable door on the other side of the traps as the direction of enemy travel.
Another option that works flawlessly is to capture a giant cave spider and tame it. Then set it up behind some fortifications. Webbed creatures are always stunned and this will trigger a cage trap.
Also note that you do not want tamed GCS. You want tamed bait animals and wild GCS. Tamed GCS suck for silk purposes, if only because chaining a hostile is a PITA