the paint on our cars is so thin that the only way I'd ever use one of those racks is if I had 3M'd every square millimeter that the rack touched the back of the car.
I've had numerous friends with paint damage on their cars (none were Subarus) from older strap-on rear bike racks. any little bit of dust that gets between the rack and the paint will act as an abrasive, so that rack would need to be on there tight. if you aren't able to run the straps between the hatch and the wing on your GR, I'm not sure how you'd get it tight without damaging the wing.
I have an 08 STI hatch, and started off putting bike racks on a factory Subaru roof rack. my heavy DH bike swayed an uncomfortable amount on the highway, due to crossbar flex, so I put a Curt hitch on my STI. a hitch-mounted bike rack is stable and easy to load, but not everyone is comfortable notching a perfectly good rear bumper cover. not to mention the aesthetics of a hitch on a STI.
if I were to go back and do it all over again I'd put a Thule or Yakima roof rack on my car.