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.

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.