hobbyfarm is correct: remove the snap ring and the wheel assy should slide right off.
However----I had one once that wouldn't come off for shucks. What I had to do was drop the spindle and then press the axle out. It's easy.
While I'm thinking about it: is your GT-275 a real wimp in reverse? Mine is, and I was wondering if that is by design rather than something wrong.
Mine just about won't make reverse turns, even with moderate throttle. Ya gotta really goose it to get it going into a reverse turn.