I would suggest going with a seller that has good feedback in selling similar items and also provides an actual photo of the item and not just a stock photo grabbed from google or wherever, you should be alright then. Managed to avoid getting any fakes from there so far.
I'd go out on a limb and say yes, as the top is pretty hard to tamper with (the atomizer is clamped on) and I've yet to encounter fake gucci ph. The glass is pretty expensive, I guess, it weighs like 2 lbs! Anyways, if worse comes to worst, eBay isn't all that bad with counterfeit items as long as you know the rules (feedback/60 days i think).