I think the bark split is normal. When I paint, I use Lime, not gypsum, as gypsum stays water soluble, lime sets up in a more permanent coating (holds up to rain). You might have luck with gypsum, but I would expect it to run during application much more easily.
As far as moisture retention, white will only reflect light to lower the likelyhood of burning, the surface area ratio of leaves to roots, as well as dryness of the air will be the factors that affect drying out of the tree.
Also, the bark is dead, and a thicker layer will help protect against burning of the living portion beneath (the cambium).