Our hamster, Rusty, who is now deceased, had an eye problem once. I didn't take him to the vet, but I did discuss it with my vet, who gave (wrong word....provided me is the better word) with antibiotic drops for his eyes to get them better, and it worked. Perhaps your vet could do the same thing for the gerbil without your having to take him in to the vet.

Good luck! With these small animals, it is so hard to know when to do what!