Should be using a calcium supplement regardless of lighting.
If really worried you can get a sort of "combination" supplement which is usually a mixture of combination, vitamin d3 and phosophorous which aside from calcium, should be balanced by the diet - but if worried, you can use all 3 to be sure.
It is possible to use too much supplement and overload and that's not good either, I gut load my live food with a variety of fruit and vegetables, and then calcium dust them. I personally don't use d3/phos mixes, just calcium. They should have all the other nutrients necessary if gut loaded.
Calcium is the most important thing, because it helps maintain bone structure, encourage bone growth, and also aids greatly with female egg production. Phosphorous allows energy to be taken from fats in order to use protein - but if you have too much phosphorous, it actually stops the calcium being absorbed. This is why I don't use a combi-vitamin, because it's all too easy to be trying to do the right thing and tip the balance, and cause problems. The balance for a leopard gecko should be an even ratio of 1:1 calcium/phos
UVB allows the animals to produce Vitamin D.
I don't know if anyone else has any scientific or practical data that says leopard geckos benefit from Vitamin D but I have never heard of it myself.