Right but ~지 used for friend or younger than me.
You can use to ask something ~지 if your age higher than someone.
~거지? = 거(을 것이다) + 지(니)?
In other word, 너도 갈 거니? ,너도 갈 거야?
and no.2 sentence 그럼, 가야지.
in the sentence, ~지 meaning is will.
The sentence("그럼, 가야지") is "right, I will go".
In other word, "그럼, 갈거야", "그럼, 갈게".
~지 have many way of use.