I did not get a serial number for GSX, only my Just Flight purchase number that does not work for GSX
It's entirely normal that a purchase number won't work as a Serial Number, because it's not a Serial Number.
It's not normal, instead, that your purchase receipt won't show a Serial Number too, because it normally does, but you must ask Justflight about this, we don't have any control on what's happening on their website.
They were short on Serial Numbers last week, so they contacted us about this, but they have been provided with new ones since the 24th so, if your order was made in the precise moment when they were out of Serial Numbers, before they added the new set, it might explain why your email didn't contained one, but they can surely provide you with one if you contact them.