No the Green Mountain IBS barrel does not fit the Great Plains Rifle or Hunter. They do make a barrel for the Lyman Trade Rifle, Lyman Deerstalker, Cabela's Hawkins, & Cabela's Carbine (all of them made by Investarms). I think the reason is, are not the GPR a double wedge rifle?

Now you could get a Great Plains Hunter Barrel with a 1-32 twist and that should fit your rifle. This would give you the opportunity to shoot conicals and even some sabots if that is what you wanted to accomplish. The Great Plains Rifle with the 1-60 twist will still be an excellent roundball shooter.