The ending "-ㄴ가요?" or "-나요?" is used when asking a question informally but politely, and also making sentences soft and gentle. Here are other endings, (Please don't be panic.^^/) 그 사람은 의사+입니까? (formal, polite,neutral) 토요일에 학교갑니까?/배가 고픕니까? 그 사람은 의사+예요?(informal, polite,neutral) 토요일에 학교가요?/배가 고파요? 그 사람은 의사+인가요?(informal, polite a bit, softer, you're curious) 토요일에 학교가나요?/배가 고픈가요? 그 사람은 의사+죠?(informal, polite little bit, you're sure more than 50%) 토요일에 학교가죠?/배가 고프죠? 그 사람은 의사+지?(informal, also not polite, you're sure more than 50%) 토요일에 학교가지?/배가 고프지? 그 사람은 의사+야?(informal, not polite, you are not sure) 토요일에 학교가?/배가 고파? In daily conversation, all above are often used in different situations depedning on the relationships with listeners who you are talking with or how much you are sure about something. \^o^/

It's difficult question.... But I think the point is what you want to know. If you want to know about Noun, you can make N+인가요?. So in this case(그 사람은 의사인가요?), you absolutely want to ask whether he is a doctor(N) or not. Therefore, you can make a question according to the information which you want to know, I think.