Using the word "gear" in this way probably first became known to the world at large when the Beatles came on the scene, and from my extensive readings on them, I know that "gear" was in use well before they made it as a band. In a 1960 letter to his father, during their early Scottish tour, he wrote, "It's gear. I've been asked for my autograph". Come to think of it, his brother Michael, who became moderately successful with his own musical group, used the stage name "Mike McGear" (like the locals wouldn't have known who he was anyway!
I've never heard anyone from anyone else using the word. Can anyone explain how the word came to be used that way, and over what period of time this was?