X GXT editor is a modding tool to modify GXT files. X GXT editor can modify both GTA SA and GTA 4 GXT files. It has been designed to work flawlessly on GXT files of both games.
The user interface is optimized to be as user friendly as possible yet still deliver advanced features for GXT editing.
He knows C++, WinAPI (I'm not sure, but any programming language) and knows structure of GXT files in this case. And what is most important has much time.