I can't find specs on the GT 8.1, but I paddle the Nomad 8.1 and I assume the volume/paddler weight range is similar to the GT 8.1.
I also am 140 pounds and also love my Kingpin.
That GT should not be too big for you; the Nomad 8.1 is the perfect size for me at 140.
I can roll my Nomad as easily as I roll my Kingpin.
Your roll might need a tune-up, and also, I think people often don't outfit their big boats as well and as tightly as their playboats. If you're too loose in your GT, you'll have trouble rolling it.
Make sure the backband is snug against you, while your feet are snug on the foot pegs, and that your hip pads are snug and your thighs aren't moving under the braces. When you move, the boat should move with you - if there's loose space then you have wasted energy and a disconnect between your movements and the boat's reactions, and that will hurt your roll.