I would have a dentist do an exam (do tell them where it is sensitive). It sounds like "a cavity" to me, since it happens when you are chewing gummy worms (a cavity on the chewing surface) and not with other sweets (which could also be root sensitivity).
ummmm is this supposed to be a serious question???? really i think you should NOT give your gerbil or any pet candy!!! think of it this way: don't feed them anything that they wouldn't encounter in the wild. Do gummy worms grow out of the ground? off trees? yah, didn't think so.