GraphicsMagick - merge images

Question

I want to create 120x240 transparent image and put some images on it with positioning. I found this: gm convert -background black \ -page +0+0 a.jpg \ -page +256+0 b.jpg \ -page +0+256 c.jpg \ -page +256+256 d.jpg \ -minify \ -mosaic output.jpg

But I need to put it to fixed-sized transparent image. How can I do it?