Silver and gold "washing", a chemical process, was done for centuries, but AFAIK, nickel plating was always done by electroplating. Remember there were batteries back before Sam Morse started clicking, and Sam Colt set off explosives with an electric charge.
Nickel plating just about everything to make it appear better became so much of a fad in the late 19th century that when W. H. Vanderbilt had to bid against Jay Gould for the New York, Chicago & St. Louis railroad, he said he paid so much that the railroad might as well have been nickel plated; the name stuck and the line was thereafter known as the Nickel Plate Road. (Some sources say the name originated earlier, but the Vanderbilt business makes a better story!)
One thing to remember about nickel electroplating is that nickel won't plate directly onto steel. So nickel electroplating of guns was done by first applying an undercladding of copper, then plating the nickel over that. If the nickel is scratched or worn, an ammonia cleaner (like Hoppes No.9) can get through the nickel and attack the copper underneath, causing the nickel to come loose and peel. Best advice is not to use ANY ammonia type cleaner (lead remover, copper remover, etc.) on a nickel plated gun.
Jim