You can use "you can't be serious" when somebody tells you something what is almost impossible and it's hard to believe in it. example: - Hey Mike! - I won 1.000.000 dollars!!! ~ you can't be serious Steve! - right, I'm just kidding :)

Yes, the meaning is very close to "You must be kidding". "You must be joking" is often used to express disapproval or disagreement in addition to disbelief. It's very possible that the speaker here is expression concern or disapproval as he/she adds: You have only known him for two months.

You're correct. It does mean "you gotta be kidding". The phrase is used when the the person does not want to believe that the other person is actually telling the truth.

You are correct. It means "you've got to be kidding" or "No way!" or "I can't believe it." It is not the same as saying something like "don't take it too seriously." You might see that expression in this kind of an exchange: Girl 1: John told me last night that I'm the most beautiful girl he's ever seen. Girl 2: Well, don't take it too seriously. He said that after drinking 6 or 7 beers!