Yeah, and the interactive seat map definitely only has GA on floors (which, I might add, are still more than available as of me typing this).
Celt - if the Ticketmaster agent on the phone actually told you there was GA behind the 500s then they are full of it and misled you big time. It might not be too late to call and complain and get your tickets exchanged for floors. I've done the upgrade thing before through Ticketmaster without too much hassle. The Rogers definitely isn't set up that way, unlike say the Molson Amp where the GA Lawn seating is indeed at the back.
Edit: I also see you said you've got seat numbers, etc... Then that's definitely not GA. Definitely call them and complain before it's too late and the actual GA tickets actually sell out!