Mentioning "그 사람 고향이 서울 아닌가요?", you have asked the difference between "-(으)ㄴ가(요)?" and "-지/죠?" for factual questions, but the point is that you should learn what 아닌가 means.

These are all factual questions relevant to your example:

A. 그 사람 고향이 서울 아니죠?

B. 그 사람 고향이 서울 아닌가요?

C. 그 사람 고향이 서울인가요?

With -지/죠, Question A shows that what the speaker believes to be true are (1) that the hometown of the/that person is not Seoul and (2) that the listener knows the truth. It also shows that the speaker wishes to confirm the truth. The listener will confirm whether Seoul is the hometown of the/that person if she/he knows the correct answer. Question A translates to "Her/His hometown is not Seoul, is it?" or "Her/His hometown is not Seoul, right?"

Question B, on the other hand, shows that the speaker firmly believes that the hometown of the/that person is Seoul and wishes to confirm the truth. You should note that I have just said "... firmly believes ... is Seoul..." not "... thinks ... is not Seoul" in consideration of the meaning of 아니다 used in some interrogative sentences. This 아니다 is defined in 한국어기초사전 as the following:

물음이나 짐작의 뜻을 나타내면서 어떤 사실이나 내용을 강하게 긍정하는 뜻을 나타내는 말.

Used to express a wonder or guess and strongly affirm a fact or statement at the same time.

※ 주로 '아닐까', '아닌가' 등과 같은 의문문으로 쓴다.

Thus, it is 아닌가 (not -ㄴ가 alone) that relates to the speaker's certainty. In some contexts (for example, when the listener knows the truth), Question B will mean nearly the same as "그 사람 고향이 서울이죠?"; I would say that Question B expresses greater certainty since it can even sound like "I don't want to admit that I'm mistaken concerning her/his hometown." Question B translates to "Isn't her/his hometown Seoul?"

The speaker will use Question C if she/he wishes to know whether Seoul is the hometown of the/that person. This is a normal interrogative sentence translating to "Is her/his hometown Seoul?"; for some reasons, it seems to the speaker that Seoul is the hometown of the/that person.

Mentioning "그 사람 고향이 서울 아닌가요?", you have asked the difference between "-(으)ㄴ가(요)?" and "-지/죠?" for factual questions, but the point is that you should learn what 아닌가 means.

Let me explain this set of factual questions including your example:

A. 그 사람 고향이 서울 아니죠?

B. 그 사람 고향이 서울 아닌가요?

C. 그 사람 고향이 서울인가요?

With -지/죠, Question A shows that what the speaker believes to be true are (1) that the hometown of the/that person is not Seoul and (2) that the listener knows the truth. It also shows that the speaker wishes to confirm the truth. The listener will confirm whether Seoul is the hometown of the/that person if she/he knows the correct answer. Question A translates to "Her/His hometown is not Seoul, is it?" or "Her/His hometown is not Seoul, right?"

Question B, on the other hand, shows that the speaker firmly believes that the hometown of the/that person is Seoul and wishes to confirm the truth. You should note that I have just said "... firmly believes ... is Seoul..." not "... thinks ... is not Seoul" in consideration of the meaning of 아니다 used in some interrogative sentences. This 아니다 is defined in 한국어기초사전 as the following:

물음이나 짐작의 뜻을 나타내면서 어떤 사실이나 내용을 강하게 긍정하는 뜻을 나타내는 말.

Used to express a wonder or guess and strongly affirm a fact or statement at the same time.

※ 주로 '아닐까', '아닌가' 등과 같은 의문문으로 쓴다.

Thus, it is 아닌가 (not -ㄴ가 alone) that relates to the speaker's certainty. In some contexts (for example, when the listener knows the truth), Question B will mean nearly the same as "그 사람 고향이 서울이죠?"; I would say that Question B expresses greater certainty since it can even sound like "I don't want to admit that I'm mistaken concerning her/his hometown." Question B translates to "Isn't her/his hometown Seoul?"

The speaker will use Question C if she/he wishes to know whether Seoul is the hometown of the/that person. This is a normal interrogative sentence translating to "Is her/his hometown Seoul?"; for some reasons, it seems to the speaker that Seoul is the hometown of the/that person.