input: [It might be...] There are several possibilities: if the bloating happens suddenly and your gerbil has overall symptoms of illness (lethargy, refusal to eat, feels cold), he probably has a very serious intestinal bacterial infection. Your gerbil needs emergency antibiotics from a vet. Warm him with a reptile heater or heating pad under one section of the tank and see the vet immediately. Wash your hands carefully after handling. if the bloating came on over a short period (days or a couple of weeks), your gerbil is female, she has not lost weight overall and seems generally healthy, it is probably an ovarian cyst.Cysts can resolve on their own, but if they burst gerbils can become ill and require antibiotics. This is a "watch and wait" condition. if the bloating was gradual and your gerbil seems to have lost weight or condition, he may have a tumor. Although little can be done for tumors, see a vet so that you rule out treatable conditions. [My Gerbil...] 
output: looks very bloated, with a large abdomen