what should I expect with this rifle at 100yds.
The limiting factors are the open iron sights and the shooters skill.
I'll presume it's the GPR, the slow twist is designed for shooting Ball. Conical projectiles require a faster twist to stabilize properly and that's the Great Plains Hunter rifle with 1-32.
That said, once you've done proper load development with the variables of Ball size, Patch thickness and find the patch Lubrication Properties that
your rifle likes with your loading technique,,(Off Hand)
,, the average shooter can get 8" maybe 6" group at 100. An experienced shooter in good physical condition that spends time Practicing and keeps his rifle and gear in good condition can bring that down to 3". Get the thing on a bench and you can get it down to 1 1/2-2",
Again it's the sights, the shooters eyes and practice.