SDL_gui is a C library that runs on top of SDL to provide a (slightly) higher level interface. It provides "widgets" in the form of C structures which are allocated via functions calls (much like GTK).
SDL_gui is a property of SDL_gui respective authors, vendors and developers. You can download SDL_gui for your hardware. If you want to use SDL_gui for other purpose, Please contact the authors, vendors and developers of SDL_gui.