Besides being used to refer to the future, -겠 can also be used to guess at something or to speculate. When the subject of a sentence is first person (I), the reply 모르겠습니다 (I guess I don't know) does not make sense because there is no reason to guess about your own lack of knowledge. You either know it or not, so instead of 모르겠습니다, it would be more logical to say 모릅니다 (I don't know) when referring to your own lack of knowledge.
However, you can speculate on someone else's lack of knowledge, so you could use 모르겠습니다 when the subject of a sentence is a third person. Consider the following dialog:
A: 그 사람 6시에 시작하는 걸 알아?In the above conversation, Koreans would normally say 모를 겁니다 when the subject is a third person, but 모르겠습니다 means the same thing and, therefore, should be able to substitute.
B: 그 사람은 (아마) 모르겠습니다. (그 사람은 모를 겁니다.)
A: Does he know it starts at 6?
B: He probably doesn't know.
I think what has happened is that Koreans used to use 모르겠습니다 only with a third-person subject, but, over time, they started misusing it with the first person (나) until it has now become pretty much standard. However, I would still recommend using 모릅니다 instead of 모르겠습니다 when the subject is first person (I) since 모릅니다 is more logical and works just fine. In other words, when someone asks you a question to which you do not know the answer, it would be better if you said 모릅니다 instead of 모르겠습니다.