Some of my reply is quite subjective regarding the differences in the subjective experiences, so some will most certainly disagree about certain things.
First some facts. GBL is a liquid, a quite corrosive liquid (although this is often somewhat exaggerated, but yes, GBL is an industrial solvent and effective for e.g. removing paint). It is extremely potent, with doses usually in the range of 1-3 ml. The difference between a nice and euphoric dose and an overdose is
very small. Few other drugs have such a small safety marginal. Only a few millileters too much will render you unconscious. Overdosing on GBL is even more dangerous if combined with CNS depressants like alcohol and benzodiazepines.
NEVER mix them!
GHB is usually comes in powder form, but also very often in liquid form. In fact, of the 50-60 times I've taken GHB, I've only once had it in a brown powder form.
Dosing should be done with extreme caution because you are seldom sure about the purity. Also, I've seen people OD on GBL because it was sold as GHB (in liquid form, GBL here is approx 5 times more potent than liquid GHB). Also, GBL has an extremely bitter taste and burns in the mouth, while GHB tastes very salty. Some say liquid GHB tastes like semen
Subjectively, I love GHB and hate GBL. GBL kicks hard and fast, in just a few minutes, and is very heavily intoxicating. Besides alcohol, GBL is the only drug on which I've seen people behave shockingly, violently, breaking things, falling around, vomiting on themselves etc. GHB has a smooth come-up, maybe 15 minutes, and feels very soft and calming, euphoric, sometimes even empathogenic, but more often people get veeery horny on it
The problem with GHB/GBL is that the purity and form varies from place to place so it's impossible to give any advice on how to do it safely.
Start low.. I managed to find a perfect "starter dose" of GHB and re-dosed every 1.5-2 hours with only half of the starting dose, and achieved exactly the same euphoria. I always followed this way of taking it and never had any problems.
Both GHB and GBL are physically addicting when used continuosly over a long period. The WD symptoms can be compared to those of benzodiazepines and alcohol; there are indeed cases where GHB addicts have been treated with heroic doses of diazepam in hospital settings. There are documented cases of Delirium Tremens from GHB withdrawal too. It seem GABAergics produce the worst physical addiction and most dangerous WD symptoms of almost all drugs (Benzo's, Barbiturares, Alcohol, GHB/GBL).
GHB is a humongously euphoric and fun drug, as long as you can control it. It may sound like a cliché, but GHB/GBL are amongst the easist drugs to overdose on. On the other hand, especially GHB, is one of the least toxic drugs there are, and used in moderation the physical/mental harm is among the lowest of all drugs (remember, GHB is naturally produced by the brain, like e.g. DMT).