A: 그건 그냥 걔라서... ---> It's just that it's him(her)... B: 그럼 걔랑 사귀든가! ---> Why don't you date him(her) then? If I'm not mistaken, 든가 is a colloquial way of writing 던가 and when glued together like in the above sentence, it means something like "why don't you..?" or "do it or not, it's up to you".