The authors of the DesignPatternsBook came to be known as the “Gang of Four.” The name of the book (“Design Patterns: Elements of Reusable Object-Oriented Software”) is too long for e-mail, so “book by the gang of four” became a shorthand name for it.
After all, it isn’t the ONLY book on patterns. That got shortened to “GOF book”, which is pretty cryptic the first time you hear it.
Source: http://c2.com/cgi/wiki?GangOfFour