Quotes someone's suggestion — the -자 "let's" form — and reports it with 그러다: 이거 남길이 형이 하자 그런 거지? "Nam Gil was the one who said let's do it, right?", 어디 가자 그러면 "if someone says let's go somewhere". Five of these lines wrap it in a condition, describing what typically happens when the suggestion gets made. The full form is 하자고 하다; dropping the 고 and using 그러다 belongs to casual speech only.