I should clarify my previous post, typed that in a hurry.
If you have a growing juvenile or a female breeder, or a rehabilitating gecko recovering from MBD, I would leave a bottle cap of pure calcium without D3 inside the tank. This is because growing geckos (under 1 year old) require additional calcium, same with a gravid female or recovering geckos.
But if you have an adult gecko, or a female that you are not breeding, you don't need additional calcium inside the tank. Repashy calcium plus is an all-in-one supplement that gives your geckos more than enough supplement needs. There's such thing as hypervitaminosis (vitamin poisoning, google it up) that too much supplement can be harmful. Geckos are very small, loading them up with too much of anything can be bad.
All in all, if you are a responsible owner, you gutload your feeders and dust 2-3x a week with multi-vitamin supplements, your geckos should not be lacking calcium or vitamins. Problems usually occur to owners that are not well educated, many of them don't gutload feeders, many don't bother dusting at all.