The power of the gauntlet is that it reveals people's weaknesses to the wearer of the gauntlet, and to many people (Rumpel and Cora, for example), love is the greatest weakness. Rumpel's weakness was his love for the thing he loved most -- power. Rumpel's power flows from the dagger at this point, so when Belle used the gauntlet to discover his greatest love (she thought it would take her directly to him), it took her directly to the real dagger instead. That's when she realized that their entire marriage had been built on a lie, and she reacted accordingly by using the real dagger to banish him from Storybrooke.