Gauntlet of Power is a mythic Artifact card from Dominaria Remastered (DMR), collector number 378. Current market price: $3.80 (foil $4.42), TCGplayer market prices via Scryfall.
As this artifact enters, choose a color. Creatures of the chosen color get +1/+1. Whenever a basic land is tapped for mana of the chosen color, its controller adds an additional one mana of that color.