Well...one thing for sure, don't use a Scotch-Bright pad on it, for it will scratch the snot out of it. Myself, I use oil soaked (0000) steel wool on the tough spots then wipe it down with an oil rag.
Ammonia won't hurt nickel, used properly. If you use a bore cleaner containing ammonia, don't soak the gun in it. Use it in the bore and wipe any excess off of the gun.
Just use a light oil to wipe the outside of the gun down.
Don't use any of the lead remover cloths on the outside of the gun.
I only use oily rags to wipe down my nickel plated guns. It gets most of the dirty markings from firing cleaned up. Once in a blue moon a brass brush in the bore.