If your gerbil isn't eating or drinking, you should get a syringe (one without a needle, obviously) and feed him on your own. You can buy baby food and put some in a syringe and feed him. Do that with water too. When one of my hamsters was sick, I took him to the vet and they told me to do that. They also gave me some medicine for my hamster. If you haven't already, go to your vet and maybe they'll give you medicine for your gerbil. Also use the syringe.