I just searched 15 minutes to find out how to resize a image using GraphicsMagick.

gm convert -resize 150x100 in.jpg out.jpg

This will resize the image in.jpg to 150x100 and saves the result in out.jpg. You can resize the image by either providing the width or the height:

gm convert -resize 150 in.jpg out.jpg
gm convert -resize x100 in.jpg out.jpg