one thing I have noticed with the GT seat is the tilt towards the front. I've had this problem with another scooter, and I fixed it the same way.
After opening the GT seat remove the two nuts holding the seat on..slip two 1/4" flat washers over the bolt under the bracket on each side...then, do the same with the two nuts holding the seat to the scooter, slip on two flat washers and reattach...the 1/4" or so doesn't make any difference to the scooter, but it certainly helps keep me from sliding forward as much.
It really is amazing how much difference that little 1/4" makes.
It's worth a try before going to all the trouble of shaving foam and reupholstering...you can do that if this doesn't work for you.
Good luck,
karlu