GBL is far too strong for caffeine to kill it, unless you're doing pretty small amounts. Dropping your GBL in a cup of coffee I found was a great way of reducing the drowsiness. I can attest to it mixing v. well with regular amphetamines. Combining it with mephedrone left me having a bit of a psychotic episode until the GBL wore off, HOWEVER I did consume a pretty heroic amount of it trying to send myself to sleep, although, (in my opinion) mephedrone is more dissociative and smashy than a true stimulant.
As usual if you can avoid mixing it with alcohol or other downers and don't fall into the trap of punching above your weight on the dose, because you are stimulated. Seems quite easy to end up in a waking sleep and go a bit nuts.
EDIT: I'd assume the usual health risks of downers + uppers apply to this situation. Unless I'm mistaken alcohol and energy drinks are much worse on the body than alcohol alone.