There's a white paste coming out of my gecko's vent. Is this calcium or something else? Should I be worried about it if the gecko isn't metabolising the calcium properly?
This is not calcium, but uric acid crystals.
All animals produce ammonia (a nitrogen compound) as a waste product. Ammonia is toxic and must be converted to something else before being excreted (with a few exceptions of aquatic animals that excrete ammonia directly into the water). Mammals excrete their waste nitrogen mainly as urea, but most reptiles excrete it as uric acid crystals. Uric acid is white and looks a bit like sand or some kind of sandy sludge.
The conversion of ammonia into uric acid is more energy-intensive than the conversion of ammonia into urea, but it is less toxic and reduces water loss and the subsequent need for water. (Urea requires large volumes of water to be dissolved in and removed from the body.) Since many reptiles developed in hot climates, this is clearly advantageous.
This article explains more about the differences between uric acid and urea (although it is primarily concerned with their function in humans and doesn't mention other animals).
So, in summary, you don't need to do anything about it: it's a completely natural and normal part of your gecko's digestive system.