I had this same problem.
I think the problem is in GPConverter.
Try using Mirko's tools and then add 4 to the picture when you pass it into the blitting function.
gp_SpriteBlitT((u16 *)gbasp+4, 0xFFFF, gbasp_width, gbasp_height, 10, 10, framebuffer);
I was able to get it to work without...