I have had two GTs - both with tan leather and both needed the leather cleaning desperately.
1) An upholstery brush
2) Autoglym leather clean
3) Autoglym Leather conditioner
sprayed the cleaner onto the rear seats so they were pretty wet. Used the upholstery brush to rub it all into the grain of the leather and then wiped down with a dry microfibre cloth. Then moved to driver's seat, repeated the process, then the passenger seat, then the armrest and door cards.
Then waited for the leather to dry and applied the leather conditioner/cream to a clean, dry microfibre cloth and began rubbing into the seats very deliberately. Leave to dry and you're good to go.
There will be a lot of responses and ultimately different people will use different products. Some say that using soap and water and some moisturising cream for use on ur face will be good and cheaper. I have tried saddlesoap and water and soap but I stick with the autoglym.