You have two main options for casting your pendants, lost wax
casting or centrifugal casting.
With lost wax casting, you could have your pieces made in either
solid brass or bronze. The advantages are lower mold costs, they
would then be solid bronze/brass, you could patina them, tumble
finish and have no need of plating.
With centrifugal casting, the parts would be cast in a zinc or
pewter alloy. Your mold fees would be higher but your per piece cost
would be lower, but you would then have the additional cost of
having them plated, then oxidized and finished. Still cheaper than
lost wax though.
If your gold cross was inexpensive, it probably just gold plated. If
there wasn’t any shiny metal plated on first (underneath) like
copper or nickel, it’ll leave the gold a matte finish.