Gauntlet of Power increases both mana and creature size for a chosen color. Creatures of that color get +1/+1, and basic lands tapped for mana of that color produce one additional mana. A mono-color deck with many basics can use the artifact to move from ordinary development into much larger turns.
The effect is shared. Opponents' creatures of the chosen color receive the bonus, and their qualifying basic lands also generate more mana. A deck using a less shared color can sometimes capture more of the benefit itself.
The land must be basic and must be tapped for mana of the chosen color. A nonbasic land with a basic land type does not qualify merely because it is a Forest or Island. The distinction makes an actual basic-heavy mana base important, rather than simply a collection of typed dual lands. Five mana is a substantial setup cost, so have something useful to do with the increased output. Expensive spells, repeated commander activations, and X spells can all provide outlets. The creature bonus can also improve a token army, giving the card two productive roles in a suitable list. Deploy it when the additional mana and combat power advance your plan faster than they advance opponents' plans, especially if you can use the bonus immediately.
