first: this software works only with default fsx airports?
second: the software is installed everywhere, that is default in every airport?
First two questions were answered on the product page:http://www.fsdreamteam.com/products_gsx.html
Purchasing GSX, will enable to use it at ALL FSX Airports, both default and 3rd party, even those not released yet.
third: The software weighs on fsx?
fourth, the minimum system requirements?
3rd and 4th questions would be better answered by downloading and installing the free Trial version, which works on all FSDT airports and a couple of default airports (LIML and EDDM), so you can test the impact on your system.
fifth where can I get?
5th question was also answered on the product page:
GSX Try-Before-Buy product. You will freely download a Demo version, that will work in full at all FSDT airports, and with a couple of default FSX airports. The Demo can be unlocked by purchasing the product, directly inside Flight Simulator
The same page also has a big orange button named "Download FSX version", which is also repeated on the main "Products" page of our website, that lists all downloads for all products.