Gold leaf is so thin, I doubt it would be able to "crack". Also, as a precious metal it is pretty flexible too and should withstand any tension from the oil film below I would think.
The question I have is what is the best a size/adhesive for gold leaf on oil? If anyone knows I would be interested in the answer.
I don't know if you have seen the current Painting with the Masters thread (Hans Memling): we have a painting with gold background and I have experimented with gold leaf on that one, I had egg tempera below though and have used an acrylic-based gilding milk because I didn't know better