I have hamsters, not gerbils...but when mine fought the vet told me to clean the wound with peroxide and put a light layer of Neosporin on it. Don't leave any residue that the gerbil could eat off.
Also, make sure everything is really clean - bedding, wheels, toys, etc. so there is less chance of infection.
Other than that, watch for swelling, infection, or an abcess forming. If it's healing properly, it really shouldn't take more than a few days to notice there is improvement. Little guys like hamsters and gerbils tend to heal quite fast.
If it doesn't seem to be getting better in the next day or two, or anything looks abnormal I would take him to the vet.