I saw it mentioned in one of the "everything you wanted to know about the making of Star Trek" books -- possibly even "The Making of Star Trek", by Stephen Whitfield. Apparently it was from a line that was cut from a script; it went along the lines of "May the Great Bird of the Galaxy roost upon your planet" and was meant as a humorous blessing. And before you know it, Gene Roddenberry was being referred to as The Great Bird of the Galaxy. (And of course, having typed this, nothing would do but that I should go and see if I could find the actual quote -- success! p. 287-288 of "The Making of Star Trek", by Stephen E. Whitfield and Gene Roddenberry. The episode in question was "The Man Trap", the line was Sulu's, and the first person to use it in reference to Gene Roddenberry was Herb Solow, whom IMDB lists as "executive in charge of production".)