How can I find a gmail addresses? I know the name, city, and other information about someone, I know it has a gmail account.

I don't think you can, other than asking around, googling for it, asking that someone, subpoena'ing Google Inc for the info, writing a letter to that address asking for the email, or trying a lot of variatons on this person's first and last name with @gmail.com appended, I guess...
Can you tell us more about the context of this question...?