Yeah, I have modded the game to permit gremlins as pets, and now they're ALWAYS available. It's pretty neat.
1) If they arrive with migrants, they will bring a crossbow and have the hunting labor enabled (I don't think you can see their labor screens to play with it right now, though), and soon they'll go hunt game. However, they don't seem to reload after running out of bolts (or maybe I just never have enough bolts to go around!) and they never carry back kills; they immediately go find something else to kill. I don't think gremlins born in your fort will go hunt. I've never seen it happen...
2) All sentient pets need to eat, drink, and sleep, unless they have the tags that disable these. [BONECARN] effectively makes a creature both a carnivore and able to eat bones (and maybe also bonemeal?) Therefore, gremlins must have fresh meat available, a water source (NOT a well; pets can't figure out how to use wells for whatever reason) and they'll basically crash wherever they're standing when they declare nap time.
3) like other sentient, CAN_SPEAK pets, gremlins will talk to each other and make friends, but not with dwarves or any other species. They can also get elected mayor, which is always awesome.
4) They can live for up to 1000 years. They can also reproduce, and their children develop a the same rate as dwarves. However, their mothers don't pick up babies, and so they can get in a little trouble occasionally.
5) Because they grow up slowly and you usually don't have many available females to start with, it's almost impossible to have problems with a gremlin-explosion. Once they max out at 50, the women will stop getting pregnant (but their current pregnancies will not be aborted.)
6) Without modding, they by default only cost 1☼. That means you can import as many gremlins as you want, or bring hundreds on embark, for very little cost to you.
7) They're only 1/6 the size of a dwarf as adults, you can't exactly arm or armor them, let alone train them. They don't exactly make very good melee infantry. However, if you've got enough hunters with crossbows and ammo they could easily make good auxilliary marksmen.