I just bought GSX after using it will my FSDT airports. I had some doubts about buying the full version.
That first sentence already doesn't make much sense: how you could have any "doubts" after trying GSX at FSDT airports, over which it's works perfectly since they are entirely customized, is really strange.
Your advertising states that GSX works with ALL airports
Yes, it does. Your test doesn't have any meaning, because the size DEPENDS on your airplane. If you tried to use an airplane on a parking it doesn't fit, that doesn't obviously means GSX "doesn't work there", it means the airplane selected was too large for that parking. Using a suitable airplane would have worked there.
That is really only true if the user makes adjustments to the airports that do not work
Only if you want to allow an airplane larger than the parking radius the scenery designer has programmed for that parking.
I expected GSX to work right out of the box, but it did not.
Yes, it does.
I am not pleased with your product. It has been a huge letdown
As explained, countless of other times on the forum, GSX cannot know, from the radius of the parking, if that radius is small because the scenery designer made a MISTAKE and there IS plenty of space available instead, or if the radius is just about right and setting it higher would result in vehicles ending inside buildings, or clashing with nearby vehicles or AI airplanes.
So, the ONLY possible choice that GSX can do, is how it works right now: compares the parking radius as specified in the AFCAD against half-wingspan of the airplane you are flying, not allowing it if it's larger than that.
I do not feel that I should have make adjustments to airports that do not work, that should have been your job
If GSX did what you might implying it should, IGNORING the parking radius and allowing any airplane in any parking regardless of the size, once you'll start seeing service vehicles that, in order to start spread over a radius that would be good to reach all doors on 747, became invisible because they started inside a building, or clashed into each other or with nearby AI or ground vehicles, the only option would be CUSTOMIZING the parking size, which is exactly what you are complaining in the first place: not having to make adjustments.
In fact, the very idea that GSX would works "out of the box" REGARDLESS if the scenery has been designed good or bad, is wrong: the only option we have is to hope an AFCAD is made correctly (if a parking spot has 30 meters of headroom it shouldn't be programmed larger than that, and if there's plenty of space available, it's best using it) and if it's not, ALLOWING some customization.
Please correct me, if I am missed something.
You missed almost everything, but after this explanation I'm fairly sure you got the reasoning behind it.