가고 있다 is present continuous. "I am going" 갈 거다 is future. "I will go" 가겠다 can also mean future, especially when used very formally to ask a question or for short, formal responses. Ex. Yes sir, I will definitely go! 네, 꼭 가겠습니다! But more often V + 겠 means something closer to "would." Ex. If it were you, would YOU go? (implying it's a bad idea, you would not) 너같으면 가겠어? Honestly, the usages, for "겠" are pretty complex and depend a lot on the context.