There are many reasons why you might want to consider making your own custom blend of gerbil food. There are a handful of great diets that are already sold and made for gerbils. We currently recommend:
Sunseed Sunscription Mouse, Rat and Gerbil
Brown's NATURAL Tropical Carnival Gourmet Food for Hamsters & Gerbils
Higgins Sunburst Hamster and Gerbil
Home made diets are really in the realm of breeders, rescuers, and people who keep more than twenty gerbils. Custom diets are exceedingly expensive for just a single pair of gerbils. You will also need to consider how you will store unused food. Using a vacuum sealer is very helpful.
In order to decide how much to use of any ingredient you will need to do some fancy math. I use an excel spreadsheet to calculate the weight, protein, fat, and fiber of each ingredient that goes into my mix.
Example Recipe for Two, ~14% Protein, ~7% Fat
Half a box of store brand/Cheerios with 0g sugar
Half a box of store brand/FiberOne Bran Sticks with 0g sugar
1 pound barley flakes
1 pound rolled oatmeal
1 pound millet
1 pound Calf Manna Pro Pellets
Half pound sunflower seeds
Half pound flaked or split peas
Half pound pumpkin seeds
Half pound long grain rice
Half pound unsalted peanuts
Half pound lentils
Additionally you can add up to a half pound of other ingredients, use small amounts (1-2 oz) of raisins, cranberries, unsweetened banana chips, freeze-dried meal worms, dehydrated veggies (carrot, bell peppers, green beans), pasta, or other seeds, nuts or grains.
There are many ingredients that can be bought in bulk or straights. Whether you buy small amounts from your local grocer or you buy 50 pound bags from the feed store. This is just a list of ingredients I've used in the past. I do not list some ingredients like milo, sorghum, safflower seeds, corn, and timothy hay pellets. These aren't really eaten by gerbils and are wasted food. They will eat corn, but its not nutritious.
Manna Pro Calf Pellets
Iams Original Cat Food
Dehydrated Bell Pepper
Dehydrated Sweet Potato
Dehydrated Green Beans
Unsweetened Banana Chips
Cereal w/ 0g sugar