I know it states that it works with default airports but I'm confused with the mention of working other airports. I will be using it only in P3d not FSX.
A purchased GSX works with EVERY FSX airport. Of course, I hope you don't expect that, by purchasing GSX, you'll get airports for free: what you'll get it's just the GSX support for them, but you will still have to purchase the airport itself.
For FSDT airports, instead, it's the opposite: if you purchase an FSDT scenery, you can use GSX for free there, even if you haven't purchased it yet. But again, purchasing GSX won't give you the FSDT airport for free.
So, basically, as the web site says, purchasing GSX will earn you the ability to use it on ANY FSX airport other than the 3 Trial airports and the FSDT ones.
