Actually it might have some effect.
GHB is a heavily GABAergic drug, MDMA doesn't effect GABA too much there there is still some overlap there. Also there may be some dopaminergic tolerance, and GHB is one of the only other drugs that releases prolactin (the orgasm hormone) and oxytocin (love hormone) as well so those effects of MDMA would likely be lessened as well. I noticed a bit of a dampening when I was abusing GHB.
it shouldn't be much, and honestly you could probably just ignore it but there should technically be at least a small amount of tolerance there. Certainly more than alcohol, but it shouldn't be extremely pronounced. Using it one time would probably do nothing but if you were using it every day for long periods of time I would expect quite a bit of overlap