Cat has a liquid filled bump on his gum?

Even if your cat is eating and drinking normally there could still be dental disease persent in your cat's mouth. Cat's don't really stop eating because it hurts, that is the last thing they will do. I would advice to see your veterinarian for an oral exam. Sometimes vet's will see animals free of charge from a oral exam / post dental exam. Is your cat up to date on their annual wellness? If not the vet will probably want to do a full annual exam. Your cat is probably experiencing some pain from this whatever it could be. It could be a number of things and may require a dental under anestesia and maybe even dental xrays. During an oral exam you will recieve an estimate of what the bill will be and what they may want to do for you. Good luck.

It's most likely an abscessed tooth. Cats don't really show pain, but that doesn't mean he's not feeling pain. He needs to see a vet, as eventually it will become very painful and he will stop eating - then you'll have a very sick cat on your hands.