When you choose a Gmail address, you actually get more than just "email@example.com." Here are two different ways you can modify your Gmail address and still get your mail:
1] Append a plus ("+") sign and any combination of words or numbers after your email address. For example, if your name was firstname.lastname@example.org, you could send mail to email@example.com or firstname.lastname@example.org.
2] Insert one or several dots (".") anywhere in your email address. Gmail doesn't recognize periods as characters in addresses -- we just ignore them. For example, you could tell people your address was email@example.com, firstname.lastname@example.org or email@example.com. (We understand that there has been some confusion about this in the past, but to settle it once and for all, you can indeed receive mail at all the variations with dots.)