The plastic nature of
straight gesso doesn't allow much penetration of watercolor.
Adding Calcium Carbonate helps make it a little more porous. The
Ox Gall breaks the surface tension of the watercolor, allowing
it to spread and penetrate better. If you cant get Ox Gall, baby
soap with a neutral pH can be used. A tiny amount is all you
need. It's effect can be seen on a plastic surface - Try
painting straight watercolor onto a piece of plastic, then add
a little baby soap and see the difference.