void GX_SetLineWidth(u8 width, u8 fmt);
I'm not exactly sure how to interpret the fmt parameter, but it's one of the following from gx.h:
#define GX_TO_ZERO 0
#define GX_TO_SIXTEENTH 1
#define GX_TO_EIGHTH 2
#define GX_TO_FOURTH 3
#define GX_TO_HALF 4
#define GX_TO_ONE 5
By default, GX_Init() sets the width like this:
GX_SetLineWidth(6,GX_TO_ZERO);
Michael