This page will tell you everything you need to know about the dietary requirements for a crested gecko.
Staple Diet: Feeding a crested gecko is very easy. Commercially prepared diets are readily available in every pet store and online, you will sometimes see them referred to as MRP (meal replacement powder) or CGD (crested gecko diet). Pangea, and Repashy are the two top brands. They come in an opaque bag in powdered form. They must be kept in a cool dark place, so we keep them in the fridge. To feed, you simply mix with water, do not feed this dry to your gecko. Some geckos prefer it a little thicker and some a little more runny, you can experiment with the ratio if your gecko is on the picky side. Pangea in particular has a formula for growth and breeding, here at Pixie Pets we primarily use that and the red flavor bag (there are many flavors to choose from). You will find some geckos hate Repashy or some hate Pangea or some just hate this flavor or that flavor. Repashy has tiny bags in pet stores and you can order a rainbow pack from Pangea directly to try a sample of each. These commercially prepared diets are complete diets and you don't actually have to use anything else if you don't choose to do so. We do recommend live insects just for variety and stimulation.
Insects: Live insects can be added to your crested gecko's diet. Never use dehydrated or you may cause constipation, impaction, and dehydration of your gecko. You also need to understand the nutritional content of each insect you plan to feed. They all have their pros and cons, but most importantly watch out for the calcium to phosphorous ratio (Ca:P). Geckos need a ratio of 2:1 minimum, most insects are the opposite, where the phosphorous far outweighs the calcium, you can see this in the chart below. This is why it is important to "gut load" your insects with calcium rich foods and dust them in calcium powder. If you need starter colonies or have no interest in farming insects but would like to provide your gecko with a variety of high quality affordable insects, you can get a 10% off discount on your entire order from Dubiaroaches.com in the Shop (*full disclosure, I get a referral bonus in the form of points to use as dollars off my future orders). Below is a chart that lists each readily available insect's nutritional content.
Wild Caught Bugs: Do not feed your gecko wild caught insects. Most insects from outdoors come with the risk of parasites of which they are only intermediary hosts for, meaning the goal of the parasite is to end up in a larger creature that will eat the bug. Some insects may have also been exposed to pesticides, you never know.
Supplements: Along with dusting insects with calcium with D3, there are many brands of commercially available multivitamins on the market. You can choose to dust your bugs in a multivitamin every other feed instead of calcium.
Calcium Sacks: A way to make sure your gecko is getting enough calcium, is to check the sacks inside their mouth. You can try to get them to open their mouth by rubbing along the throat and sides of mouth. Sometimes a gecko will get annoyed and show you their fierce silent scream, sometimes it takes more work and you may have to gently pry open their mouth. Sacks and are on the roof, a bit towards the back of the mouth, they should be white, plump, and round, sometimes a little oblong. The two sacks should be touching each other in the middle or almost touching. I'd say they're maybe 1/2 to 2/3 the size of a tic tac.
Feeding Schedule: Baby geckos need fed every day in smaller amounts, while adult geckos need fed every 2 to 3 days in larger amounts. We tend to feed babies 1/4tsp dry measured powder and adults 1/2tsp (1/4 red flavor, 1/4 growth and breeding). For insects, we feed the babies 2-4 insects every other day depending on type of insect and we feed the adults 4-6 insects once or twice a week. Some geckos have very high pray drives and might need live food more often, some have zero interest in bugs and can go with less but we always offer anyway.
Fruits: Crested Geckos can eat some fruits, we only recommend fruits as treats, never as the staple diet, it is too hard to come up with a nutritionally balanced diet. Here at Pixie Pets, we do not feed fruits at all, there is no benefit over the prepared diet and you may actually cause them to just want fruit and not eat the prepared diet and go on a hunger strike. Since we don't feed them, I do not have a complete list of good fruits vs bad, however never feed dried fruits like raisins, hard fruits like apples, or citrus like oranges, lemons, or pineapples.
Water: Geckos will drink from the dropplets that misting every day provides, but you may also add a small dish of water, some of them will drink from a bowl. The dish should be the same size as the feeding bowl (about the size of a juice bottle cap). All water, including for misting should be chlorine free, which means buying it that way or adding a conditioning product to remove it.