That wasn't referring to GCS in particular, just the interesting cavern creatures like giant toads, cave crocs, etc. I do have a GCS though, might have been about the fourth one to turn up. I guess that may not be a direct comparison, but if the game has rolled eight large predators then I'd start getting a little antsy about not having a GCS. How many of the large beasties have turned up?
What I do for collection is seal off some entrances with fortifications (the theory is that because they don't create a floor above them, there's no spawning spot) and seal off some of them with walls a little further in, so that creatures can spawn but have nowhere to go. Once a creature is selected for spawning, I check it out via the Units list. If it's one I want to catch, I check that traps are in place and deconstruct the wall. If it's one that I don't want to catch, I station the military by the wall before deconstructing it. Because there are multiple entrances, there's a fairly good chance that the next creature(s) to turn up will do so somewhere else. If they don't, that can be unpleasant depending on what turns up. If it's more crundles, that's fine, I wanted them dead anyway. If it's something big like a jabberer, hopefully I have enough time to call off the military before they kill it. If it's a GCS, that's bad, because it'll either kill the military, bounce its fangs off their helmets, or get killed by a counter. If it's a trio of blind cave ogres, that's... fine in this particular case, but my soldiers are pretty good.
A design I'd like to try is something like this:
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The only way for creatures to get from the map edge to anywhere is to follow the corridor. First there is a door, which when locked will keep most creatures out. Then there is an empty section of corridor. Then there is a wall, which will keep anything out. Then there is one trap. Then an empty section of corridor, and finally some backup traps. The whole thing should have a ceiling, with the 'floor' at the edge of the map being made of fortifications.
Most of this is insurance against creatures turning up in the same spot twice. The aim is to deconstruct the wall, draw an unwanted group past the door, get it locked, and kill them in the corridor before they can use up any of the traps. If it's a wanted group, simply deconstruct the wall and keep the military on standby. Odds are they're some kind of predator and chase down the dwarf like a T-Rex chasing a jeep full of tasty scientists, and then they'll end up trapped.
If the next group appears in another entrance, repeat the process. If it appears in the same one, the door will either keep it out entirely or buy enough time to either pull everyone out or rebuild the wall / reset the traps (risky).
You can use a raising bridge (or retracting bridge over a ramp) in place of the wall, or a hatch cover over a ramp. Anything that's completely impassable to creatures will do.
It's pretty labour-intensive on your part and you'll be doing a lot of pausing and manual managing, but as far as the dwarves are concerned they're spending the whole time running around. Just keep an eye on what's going on elsewhere, and have a good store of food and drink and clothes and a generally smooth-running fortress. Maybe stop all non-essential production and hauling, and ignore traders unless you have a cache of aluminium goblets or silver serrated discs right by the depot. If you're trying for any large cavern creatures, you have something of a limit based on the lifespan and current age of whatever you catch. GCS currently don't breed, but can be quite easily modded to. If it's only the GCS you're after, then the only limit is your patience for cutting through unwanted creatures.
I don't have any distractions right now. The only two civs with access are dwarves and goblins, and the dwarven caravan tends to crash the game so I disabled the active season on dwarves. I was halfway through a streamlining mod and getting fed up with the untraceable crashes at worldgen I was getting, so I tried a world that looked interesting (it wasn't). So the fortress has no surface activity at all, getting only never-revealed ambushes.