I just wanted to make a post in regards to this misunderstood topic. It is a very complex topic and I will not go deeply into the details of how calcium, d3, and phosphorous all work and how they effect each other, but I will give a basic explanation of why each one is necessary.
I get at least one phone call or email per week about someones gecko that is showing signs of severe calcium deficiency, and after asking a few questions I find that it is because they are feeding insects that are not supplemented with cal and d3 or they are only supplemented with calcium. In some case people think that using certain gut loads is enough. (it's not)
If you feed strictly Crested Gecko Diet, you have nothing to worry about, it contains the correct amounts of calcium and vitamin d3.
However when you add anything else to a geckos diet, insects, fruits, or whatever, the balance of calcium, phosphorous, and vitamin d3 all change. Insects are very high in phophorous and since most reptiles require a 2:1 ratio of calcium to phosphorous, these levels quickly become imbalanced. The solution is to dust the insects with calcium to restore the 2:1 ratio of calcium to phosphorous. But in order for the gecko to metabolize the calcium they need vitamin d3. Reptiles exposed to UVB can manufacture their own vitamin d3, crested geckos may be able to but there are not any good studies to see whether or not they utilize UVB rays or if they are even able to manufacture it. This can be argued about but the fact is most people do not provide UVB and Crested Geckos obviously do just fine without it as long as d3 is supplemented in the diet.
D3 is a fat soluble vitamin, so it can be overdosed, and this is why people freak out about supplementing it and why there are so many posts saying "never use calcium supplements with d3" and so on. I have never heard of a substantiated case of vitamin d3 overdose in any reptile (and by substantiated I mean a vet conducted blood tests, not someone blaming a mysterious death on d3), but I do hear of calcium deficiency and metabolic bone disease almost every day. A gecko would have to ingest a ton of d3 over an extended period of time before it built up to toxic levels, and commercial calcium supplements simply don't contain enough d3 to cause overdose when they are used properly. Certainly dusting a dozen or two crickets a week with a commercial calcium and d3 suplement will not even come close to causing an overdose. For those that are still worried about it, Allen Repashy makes a calcium supplement with varying levels of d3, low, medium, and high. I personally use the Medium d3 calcium supplement 2-3 times per week with every single insect feeding, from hatchling geckos to adults.
I am posting this to help people, not to start a debate, or to ruffle anyones feathers. I truly feel that the demonization of d3 is causing many geckos to suffer and often times die. The saddest part about it is that it is totally avoidable.
Keep in mind that a gecko must have a healthy intestinal track, kidneys, and liver to properly absorb calcium and particulary vitamin d3. A heavy parasite load, infections, etc, can cause nutritional problems even if they are fed a proper diet.
Does CGD have enough d3 to make up for the occasional insect feeding? Possibly if you only feed a couple of insects per week.