Things you need to get started
A wood ramrod comes with the GPR, but I would suggest a synthetic replacement ramrod. (it won't break, and it bounces nicely when the ball it seated)
Many stores that stock BP supplies sell starter kits. A kit usually has a Ball starter, cleaning jag, chamber scraper, capper, powder measure, powder flask, etc. Find one that you like.(take a BP shooter w/ you, or ask the clerk if they have BP experience.) They sell a nipple wrench that has a nipple pick that screws in the top...(is handy) Get a bag or tackle box to arrange your gear. I have a possible bag and a toolbox that I resupply out of at the range.
I like the Ted Cash capper. It holds almost 100 caps, so you load it a couple times a year and it is enclosed. the stick capper will get you started, but you will want more.
Really, powder, patch, and ball is all you need... .530 ball and a cotton patch. - I have been using a lubed patch - like bore butter.
I clean with a 2.5" patch and 409 (its just soapy water) let her dry and apply bore butter in the bore. use gunoil or RIG on the outside.
Good choice in Rifles, I have 2... 1 caplock, 1 flintlock. The GPR is a nice accurate 2 wedge muzzleloader. Shoots nice, cleans nice. It will do whatever you want to do.
Oh, the CO2 unloaders are handy if you dryball. just blow it out...I have tapped powder in the flash hole before, it works...no hairspray for my rifle!