Maybe it's the resolution : Look in init.c, at beginning of Set_all_video_modes(void) :
#if defined(__GP2X__)
// Native GP2X resolution
Set_video_mode( 320,240,0, 1);
#else
// Window mode, with default size of 640x480
Set_video_mode( 640,480,0, 0);
#endif
Set_video_mode(...