Gmail cannot recognize dots or periods in email addresses or the user names - that means an address like "email@example.com" is the same as "firstname.lastname@example.org" or "email@example.com".
If a Gmail account like "firstname.lastname@example.org" already exists, registration of "email@example.com" or any address with string "abcxyz"(charters may be separated with random pattern or sequence of dots) would fail.
This is actually a bug than a feature but you can again exploit it to have different kind of email address in your forum signatures.
Keep in mind that hyphens (-) and underscores (_) can't be used in a Gmail address. Also, gmail user names are case insensitive, so it doesn't matter if you enter upper case or lower case letters.