ㄴ가 means question. For example, 너는 바보인가? 거기로 가야하는가? 해서 means cause. Actually in that sentence, 물어봤어 is omitted in latter part Full sentence is 걔가 그렇게 대단한가 해서 물어봤어 The reason speaker asked is that speaker is curious whether he is so famous. 해서 marks the previous words are the reason of the next words For example 배고파서 밥을 먹었다, 슬퍼서 눈물이 났다

Is he so famous in the world?