Assuming you already have two images called:
- top.png (image at the top spanning a row)
- left.png (other image at bottom left)
first of all you have to create the third missing image (called text.png) containing your text:
gm convert -size 200x300 xc:#7F0000 -pointsize 80 -font Arial -fill white -draw "text 10,150 'text'" text.png
then you can compose the two images in the bottom part (creating an image called bottom.png):
gm montage left.png -geometry 200x300+0+0 text.png -tile 2x1 bottom.png
and finally you can compose upper and lower parts with:
gm montage top.png -geometry 400x300+0+0 bottom.png -tile 1x2 result.png
the result will be an image called result.png, size: 400x600
top.png:
left.png:
result.png:
Tested on Windows with GraphicsMagick 1.3.18 2013-03-10 Q8