Gerbiling refers to an unsubstantiated sexual practice in which live animals, such as gerbils, mice, hamsters or rats, are inserted into the human rectum for the purpose of sexual pleasure. Although there have been a few attempts to verify whether gerbiling is a real practice, to date most people believe it's an urban legend as no first-hand accounts have been obtained, nor is there any peer reviewed medical literature describing gerbils as one of the many other foreign objects that are removed from people's rectums in emergency rooms every year.
It is also believed the gerbiling is a homophobic legend designed to denigrate gay men and anal play in general.
Bottom line? It is highly unlikely that anyone has put a gerbil in their ass; the act may even be impossible.