I change my gerbil mix frequently, depending on whats on sale and such, but this may give you an idea.
Raisins, Apple, Cranberries, Blueberry, Coconut, Raspberries, Papaya, Pumpkin, Banana, Pineapple
Carrots, peas, Parsley, Alfalfa leaves
Oat kernels, Millet (2 types), Pumpkin, Safflower, Sunflower, Rose hips, corn, barley
Peanuts, almonds, walnuts
Cereals & Other
Corn flakes, wheat flakes, Barley Flakes, Oat groats, Wheat bran, cornflower, Soya bean meal, Extruded rabbit pellets, Cat food, Alfalfa/Timothy (hay and sometimes cubes)
Cheese, Applesauce, Cranberry juice, Lettuce (Romaine, or green), Tomato, Carrots, Broccoli, grapes, apple, clover, boiled egg, cucumber, Dog Cookie, spray millet, Cat/Kitten food, Dried corn on the cob, honey bars, Frozen Fruits & veggies (summer treat)
For Packaged feeds I like Ecotrition and I am just trying Gerri Gerbil.
Even with pre-mixes I always add some cat food for extra protine. I used to not have sunflower seeds in my diets, but have added them back b/c my gerbils are so healthy and fit they could use that little extra fat