I am guessing that you just have to slide the cams over the mast from the beginning. This is pretty standard for small luff sleeve sails like the GTX.
Rigging with the cambers off the mast and popping them on after you down haul and out haul is for the large luff sleeve, 4 cam race sails.
I think you are just going to take the time to work the cambers up the mast. Lubricating your mast with a little McLube (not sure about the spelling) spray may help. In time, the sail material will soften a bit and it will get easier to rig.