-ㄴ다 is the plain statement form and 그러다 quotes it — the colloquial stand-in for -다고 하다 'say that': 아까 단 거 안 먹는다 그러지 않았어요? 'didn't you say earlier you don't eat sweet things?', 소녀시대 조금 늦는다 그러면 'if Girls' Generation say they'll be a bit late'. It also sets up a hypothetical: 세 끼를 먹는다 그러면 'supposing you eat three meals'.