The question How can I recover my Gmail account when I remember the password, not the username? is really about an odd case where a user remembers his password, but not his username. I have suggested a title edit to reflect this.
There are other questions which are about people forgetting their Google passwords, which have been marked as a duplicate of this one, e.g. What do I do if I forgot my Gmail password?
I suggest the duplicate marking is removed, so that we keep the forgot username and forgot password issues separate.