It is, but know that GABA doesn't pass the blood/brain barrier in the vast majority of people, so supplementing with it is almost always useless. The best way to enhance GABA is to take taurine and B6, which along with cofactors produce GABA in the brain. Now, if you take benzos, then that would be a problem, as benzos target GABA, but Celexa targets serotonin.
ask your pharmacist - they will know !!! just call them up
First, pharmacists don't know anything about natural remedies. They don't get trained in them. Second, yes, if you're on Ativan, don't take GABA -- the Ativan is already targeting GABA. Don't take the taurine either. There are ways to take natural relaxants along with benzos, but if you're not expert in natural medicine I wouldn't play around.