There are commercial diets available for gerbils that you can use. However, these diets tend to have a high content of sunflower seeds, which are very fattening. It is a good idea that you try to get one that contain less sunflower seeds, or pick out the sunflower seeds from the diet. You can save this are give them as treats (a great way to tame them!). Also, you should supplement this diet with fresh or dried fruits and veggies. I have found that my gerbils like apple, carrots, grapes, raisins, different types of dried fruits, banana, etc. Also, you can offer them cereals like Cheerios or Rice Krispies (avoid cereals with sugar added).
Remember that your gerbils should always have fresh water available. This can be provided in a dish or a water bottle. A water dish usually ends up covered in litter, so it is way better to get a bottle for them.