2.5g is way too much. 0.5-1g should do for a start. It's also not really going to feel like MDMA so if that's your goal, beware that no dose will achieve that.
I recommend you take a read through the Psychonaut GHB wiki before using for dosing guidelines and to learn about the potential harms and risks of use before you contemplate taking any. It won't take 5 mins of time: