I think there is one question we really need to answer....
Is metal just a sound or the spirit of the musicians?
AC/DC is always the big question mark. In my opinion their sound is unequivocally hard rock but their spirit is totally metal. They transcend that boundary.
Same with GnR in many ways as well.
Good question.
I think AC/DC is 100% hard rock. They do have a punk rock / metal sensibility - there's no doubt. But metal has to be about the sound. AC/DC - much like Led Zeppelin - has a sound deeply rooted in blues chord progressions. The blues inform a lot of hard rock, but the blues don't really shape the sound of any major metal bands that I can think of off-hand.
Guns 'N' Roses is a little more difficult, because they have riffs that definitely feature a metal sound (Welcome to the Jungle comes to mind). However, almost every Roses song I hear makes me think of hard rock - not metal.