If Gauntlet of Power is somehow on the battlefield with no color chosen for its first ability, no creatures get +1/+1 and its last ability never triggers. It still won't affect colorless creatures or basic lands tapped for colorless mana.

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to a creature of the chosen color may become lethal if Gauntlet of Power leaves the battlefield during that turn.