A high-quality commercial pellet made specifically for gerbils, and fresh water, is the best diet. Rodents enjoy seeds, nuts, fruits and vegetables, but these should be offered in small amounts since your pet gerbil is relatively sedentary when compared to his relatives in the wild.
Gerbils are rodents and their front teeth, called incisors, are continually growing and must be worn to the proper length by gnawing. Chew sticks or pieces of other non-toxic wood are a good way to keep your rodent's teeth healthy.
For more information, please read the story Feeding Your Gerbil.