Even when the tail detaches by itself, what is often left is exposed bone (and nerve!) which can be very painful to the animal and prone to infection. There are two schools of thought on this amongst gerbil owners – that self-amputation of the tail is "no big deal" and will heal naturally and without interference. These gerbil owners have never even heard of an amputated tail becoming infected. The other school of thought is that "I wouldn’t want my bone exposed so I don’t want my gerbil in that condition".

However, it doesn’t sound like your gerbil’s tail is going to amputate on its own anyway, and like I said this is not necessarily trouble free in any case. Therefore, you need to take him to the vet, who will probably amputate the injured area and cover the end with skin by suturing.