There are different domains where you can decide if your application runs in fullscreen mode or not.
1) The "X-Borders" are drawn by your window manager. An application running on your X-Server without a window manager won't have any borders. If theres a fullscreen mode for the application depends on the window manager itself.
For example in my i3 you can use the key combination ALT + F to switch to fullscreen mode.
2) The used graphic driver behind your library.
GRX supports different drivers and you can set them with the following function:
int GrSetDriver(char *drvspec);
The SDL driver supports a windowed mode and a fullscreen mode.
So
sdl::fs
should set your application in fullscreen mode.
See the manual for more information:
http://grx.gnu.de/grx248um.html#Setting-the-graphics-driver