Said aloud, the plain -ㄴ다 form announces an action as it happens, and doubled 간다 간다 is the standard shout before you set off, throw, or launch into something. It works for yourself (오빠 간다, here I come) and for someone else moving off. 간다 간다 하고 튕기면 mocks a person who keeps announcing they're going and then hangs back. This is a shout, not polite speech.