I've contacted support asking this, and they said you cannot buy it. I bought Guitar Pro 6 and love it, but prefer Guitar Pro 5 for different reasons. I do love all the hotkeys in GP6 though, but I prefer the RSE Soundbanks in GP5 for the mostpart.
Because it's only a demo, I'm assuming the production quality is going to be low and the songs still need polish. I would only sell it if it was at least an EP. Make CD's to give away at shows. I say you should just buy a stack of blank CD's and burn to the demo to that. For artwork, you can make something so you don't just have a blank space, but you could always just put the CD in those cheap paper CD cases and write info on the back of them (band name, tracklist, etc.) but until you record a nice EP this is just something to show people what you sound like and what you are.