My gerbil have bitten me before, and I think they do shed the tip...never realised it tho

Gerbils, generally, aren't big nippers. Any animal can bite. It's up to you, as a pet owner, to teach them that biting will not be tolerated. Gerbils tend to be easier to tame than say, a hamster. Only one of my gerbils has drawn blood, twice. I'm pretty sure it was my fault, though. I probably scared her really bad. However, now she's a total sweetie. She puts up with me handling her every day and giving her kisses. She has never drawn blood since. Sometimes they get a little testy and will nip to be put down, but it doesn't hurt. It's just slight pressure.

Gerbils don't "shed" the tips of their tail, I don't think. I've never heard of this. However, their tails are very fragile. They can easily be broken or degloved, meaning the skin gets pulled off. They usually recover fine from these injuries, but you should never, ever pull on their tail or hold them by their tail. If your gerbils have stumpy tails, I would say they were in some type of accident or held improperly.