Most commercial hamster/gerbil food is designed for HAMSTERS...and will contain a mix of sunflower seeds, peanuts...millet...oats...etc.  Your gerbils have an instinct to gorge on high fat foods and will eat out all of the sunflower seeds, peanuts...and high-fat seeds first...and will only eat the millet, oats and green pellets when everything better is gone. If you feed your gerbils "hamster food" daily...you'll soon have plump gerbils.