-아야/어야 marks the condition without which the thing simply does not happen, so this is 'you only go if...': 불러야 가죠 (I can only go if I'm invited), 성적이 제일 좋아야 갈 수 있는 대학교 (the university you can only get into with the very best grades). 그래야 갑니다 does the same with 그렇다 — 'that's the only way it happens'. Compare -(으)면 가다, which states a condition without that exclusive force.