i'd also like to know about these attachments? I put that hidden hitch on my 96 impreza 2.2L awd wagon with out them and have towed my seadoo around a few times. and just got back from Ma this weekend with a uhaul trailer with out any problems. My hitch which is the exact same hitch as that same company and everything bolted on. it took a little struggle to get the holes to line up. I had to bolt in one side and pry the other side to line up it was a little short. It bolts directly through the body into the rear bumper beam. using your stock bumper beam bolts and takes out the exhaust hanger on the outer edge and replaces it with a new bolt. all in all i'd say im pretty happy with the hitch. Only thing i have to complain about is the clearance between the hitch and my sti catback can't even fit a finger inbetween them and sometimes i notice extra vibration from the exhaust.
Last year of the GC, then. It would be nice if it would fit, but I doubt it would without modification, which you really don't want to do to something as important as a trailer hitch. Unless a guru speaks up soon, I'd just call a dealership and ask what they recommend.