When I had that problem on my GT a few years back, I managed to get it working normally again just by cleaning lubricating the cables and the places they connect to the gearbox. If you get someone to move the gearstick between the gears while you watch the moving parts on the gearbox (shine a torch between the battery and the bulkhead). I found mine to be caked in black sticky road crud which I cleaned off, then I lubricated the moving parts and cable inners with silicone spray (I used an aerosol with a "straw" attachment to aim the spray in the required places). This freed up the movement of the gearstick allowing it to reach the extreme right where rear is to be found (self-centring improved also). Good luck!