GNU GPL is a software licence, not an art copyright. If an artist really used GNU as a template, then the artist already made a mistake - or it could also be that the art itself is under a different copyright and the GPL-Link was only provided for one specific use of the artwork, not for the artwork itself.
I think you should give a link to the artists in question for us to check.
Otherwise, artwork is usually copyrighted under different forms of other (non-GNU) common licences, and there are a lot of different variations giving different rights to change, use or handle the artwork.
EDIT:
Rereading your post, could it be that you haven't found an artist and have just assumed that GNU would cover artwork?
It usually doesn't and I don't know a single artist who used a link to GNU as a licence, because for art, you need different forms for licences...