Darren, I am somewhat surprised that a an ol' Muzzleloader like yourself is having problems, especially with the GPR.....but, I have heard it before, and even know of fellas trading off their GPR because of it.
I suspect my dealings with the GPR have all been very positive.....straight out-of-the-box accuracy with only a bit of experimenting.
My own GPR was "pretty good" right out of the box. But after a dozen or so fire-lappings it was never a problem no matter what the powder charge, seemed to have always had a preference for the .530 swaged and a .012 pillow tick patch. To say one lube was more accurate, from spit-patch to Moose-Milk, to Hoppes would be splitting some mighty fine hairs, but Hoppes did, and still does, seem to give me more shots in between wippings.
Something to think about;
For Fire Lappings I simply use a patch well soaked in Mineral Oil, with a large dab of #11 Valve Grinding Compound on it, load the ball with the Compound to the outside, touching the barrel, ram it down over about 50 - 55gr FFFg, and touch it off....then clean up your mess in the barrel. (The reason for the small powder charge is because it seems to create less of a mess to have to clean up).
Do this a dozen or so times, maybe even more if you have the time and patience, and your barrel will sparkle like new money, and most likely shoot a little better.
I amd also told a very tight heavy cotton patch, with the same concoction, and 50 or more strokes between cleaning will do the same thing.
The very best way to lap is with a conical lube grooved bullet, .001 overbore size, (Mini or Maxi) ...then mount the conical to the base of a good stout dowel rod, with the compound embedded in the grooves by use of a heavy lube like Bore Butter or 1000 Natura,....... using constant long, even strokes, run the slug in and out of the bore a couple of dozen times....clean the barrel good with Mineral oil, bump-up another slug and go at it again.
I hope you get that thing shooting. The GPR is one of my all time favorite toys.
Uncle Russ...