I have never had problems when taking ghb when I'm already almost completely down from E other than what you'd have when taking ghb on it's own (some ppl call it hornyness, in my case it's just innability to say no). Ghb relaxes me, it kills the hype of the mdma but for me, the happy , serene feeling is still there.
On the other hand, never ever take E after a ghb high, the two fight each other and if you're like me, you'll have the angst bad trip from hell.