"(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"

ALSO:

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.