the airport in default p3d is just a square box. so i will try to improve it with ADE. the question is , do i have to do something special after i finish with ade so it will work with gsx?
I don't understand how this could have anything to do with GSX, which only add ground support vehicles. It surely not supposed to change the appearance of airports, that's something addon sceneries will do.
2. and no , the default airport does not work with gsx
Yes, it does.
i face this problem: when i call the gsx services ctrl f12 i get this message: this parking is isolated, ground services are not available.
I checked the default LGSA airport and yes, that airport is unusable as it is, since it has only ONE parking, of the type "Fuel", which is also not connected to any taxiway. This would have been obvious if you just had a look at the airport with ADE, or if you chose the "Fault Finder" too, which reported that only parking as "Orphaned", which is the same thing as "Isolated" for GSX.
So, you just cannot say "GSX doesn't work in all airports", just because the ONE you tried is an incomplete one, with a single orphaned fuel parking.
If you want to use GSX there, you'll probably have to add at least ONE airplane parking, and connect it to the rest of the taxiway layout.
funny things is that 1st, gsx took me there (there is only one parking at default lgsa). so in the beggining gsx tells you, that there is a parking for you, and when it takes me there, i cant use the gsx services.
And why GSX should have prevented you to just move to the parking ? It's entirely normal it would complain of an isolated parking only when you call its services because, an isolated parking would prevent the vehicles finding their routes, but there wouldn't be any reason for prevent you to just move there.
4. i also noticed in some airports when i use the pmdg 747 that gsx says there is not suitable parking for that plane in that airport. i guess they dont have any gates for heavies. so also there gsx doesnt work and i would like to fix that. (what can i do? i love my 747 and i want to fly and gsx-park it everywhere) 
Why you keep saying "GSX doesn't work" ? Have you even read the manual ? If you are trying to park on an airport that doesn't have any parking spots suitable for your airplane, GSX CORRECTLY prevents you to select them, because it would likely have issue moving vehicles around a very crammed space.
But of course, GSX being the flexible program that is, allows you to customize lots of things, without having to use ADE.
Have a read of the GSX manual, Page 31, the explanation of the
Max Allowed Wingspan
