"(으)ㄹ게 = when you want to hear a response or reaction from the other person
(으)ㄹ거야 when you are determined to do something, you decide and you probably won't change your mind"
갈거야 has the meaning of one's free will (나는 오늘 밤 그 파티에 갈거야. = I'll go to the party tonight.)
but 갈게 can be used as an answer when people ask you something like this: A: 오늘 밤 내 파티에 와줄 수 있어? 내가 친구가 별로 없어. Could you come to my party tonight? Because I don' t have many friends. B: 갈게. I'll come/go.