The Gold Leaf is an item that first appeared in the Generation II Pokémon games. It does not have any purpose except for to be sold.
In the Generation II games, this item can be held by Pokémon 100% from the Time Capsule with them being Raichu, Parasect, Venomoth, Golduck, Primeape, Slowbro, Dewgong, Muk, Hypno, Marowak, Scyther Rhydon, and Seadra.