Have you actually touched a '07 GT? There isn't a piece of metal on it until you reach the engine/ frame.
The fuel tank is plastic as is the tank cover (the outside that you touch).
BW offers a bag that slides onto a bracket attached to your fuel filler ring. Nice but BMW $.
I'd suggest a bag which has the ladderlock attachments. https://www.google.com/#q=ladderlock+buckle
You can buy like ladderlock/buckles and extra web strapping to wrap around the front steering head and then another to under the front seat to the seat height adjustment bar.
Both can be tucked away when not in use.