Until recently, if your Gmail username included one or more dots, you had to add them to your username when logging in. Now it's no longer required to include the dots, so you can type johndoe instead of john.doe.
It's important to keep in mind that Gmail ignores the dots from your username when receiving a message, so you'll get all the messages sent to email@example.com, firstname.lastname@example.org, email@example.com and other similar addresses. The requirement to enter the same username you used when you created the Gmail account might have helped you keep the account secure, but it was really annoying.
There's an interesting urban legend about the dots from Gmail usernames: some people claim that Gmail initially allowed users to create different accounts using usernames that were identical if you ignored the dots. That's why you'll hear many users complaining that they receive someone else's mail. Google even created a page which debunks the myth, but that doesn't stop users from talking about the 'dot problem'.