Thankfully gerbils do not naturally develop a bald head with age like human males can. This is likely to be an allergic reaction and I have witnessed similar circumstances in hamsters. While it may take several months to see an improvement, I suggest you gradually change your gerbil’s diet (see resources below) and the cage substrate, as well the bedding. It is possible your gerbil is allergic to one or more of the elements in his environment. With all cases like this, it is probably a good idea to have your gerbil checked out by your vet, who may suggest a topical antibiotic application as well as the instructions I have given.