I guess ya could try it, but I suggest 3F real BP in the pan & in the barrel if it is under a .54 cal or smaller. I would not waste my time with anything but real BP in a GPR as they shoot Wonderfully with real BP in them.
IMHO, If it is a .54 cal, I would try 65 grains of FF or FFF, then 70 grains then 75 grains & find the best group. .535 ball, .015 oxjoke patch & some Lehigh Lube, only us 1/2 pan of powder. You need the vent to be coned inside & outside & about a 5/64" dia vent hole. Load it & then take a vent pick or a curved large gem clip & open the vent hole back to the main charge so the vent is open hole all the way. After shooting swab with a jag & a wet tee shirt patch down & back one stroke, then wipe off the flint & frizzen & wipe out the pan, reload & do it all again....
For deer hunting you can use 75 grains of same or 2F. I would not continously shoot 75 grains of 3F in it as it may eventually gas cut the barrel, but once in a while will not hurt it..
Good luck !