GT supports both message- and function-based interfaces. Most of the time, the function-based interface is handier, since most Skandha4 applications simply run on a graphics workstation and want to display on the current screen.
However, the driver library exports class GRAPHICS-TERMINAL. By creating instances of this class, selecting the terminal type for them, and sending messages to the instance of your choice, it is possible to do sophisticated things like simultaneous graphics on several different types of devices. Since nobody has done this much yet, you will likely encounter sophisticated bugs ... :)
What the graphics terminal librar does internally is essentially to set up a default instance of GRAPHICS-TERMINAL, and translate all function calls into messages to that default instance.