You said you've got your fork figured out, but i'll just post this for info. You can absolutely use a 1" steerer on a 1 1/8" tube, you just need a reducer like this:
I'm surprised a shop wouldn't have recommended that. You would just need to bring over the 1" headset from the other bike as well. They may have said you have to use a 1 1/8" threadless because that's what headset that comes with the GT2a.
As for tires, on my GT2a-s I've got 2.3" Duro cruiser tires mounted on DH39 rims which are 1.5" wide. Chain clearance is an issue, don't think I could get away with any wider without moving the engine outboard. I have a couple telltale grease marks on my sidewalls. I am running a 4 stroke though so sprocket placement might be different than yours.