The minimal library is, well, minimal

So all you get is a frame buffer, and your expected to do all your own drawing routines/etc.
So, something like:
int x,y;
for (y = 0; y < 240; y ++)
for (x = 0; x < 320; x ++)
gp2x_video_RGB[0].screen16[(y*320)+x] = 0;
gp2x_video_RGB_flip(0);
Should get you started by blanking the display. Since the display is double buffered by default, the flip routine simply swaps the buffers around, making the buffer you've just filled in become the currently displayed one.