I'm not sure I fully understand your query, so apologies if I've misunderstood, but are you trying to create a transparent image, with some text in the corner? If so, would this not work?
convert -background transparent -fill grey -font Calibri -size 140x80 -pointsize 14 -gravity southeast label:'copyright text' output.png
Obviously adjusting the font, pointsize, output image name etc. That would create the following:
http://oi42.tinypic.com/14j1bvp.jpg
P.S. that was written for ImageMagick. I don't know how GM differs or whether it would still work.