I typed:
gm convert -mosaic a.jpg -page 100x100+0+0 b.jpg -page 100x100+100+0 c.jpg -page 100x100+0+100 d.jpg -page 100x100+100+100 e.png
which is wrong in several ways. But, gm didn't generate the output file e.png, nor did it print any error message. It just failed silently. I eventually figured out the right format for the arguments here, but it seems to me that gm should at least print...
2007-10-07 14:42:01 UTC in GraphicsMagick