Gold Leaf is an item that only appears in the Generation II Pokémon games. It can be sold for 
1000 and doesn't have any other additional effect.
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, Rhydon, and Seadra.