Plaza of Harmony is a rare Land card from Ravnica Allegiance Promos (PRNA), collector number 254p. Current market price: $0.43 (foil $6.97), TCGplayer market prices via Scryfall.
When this land enters, if you control two or more Gates, you gain 3 life. {T}: Add {C}. {T}: Add one mana of any type that a Gate you control could produce.