Why is it a joke?

Gaffer within the motion picture industry originally related to the moving of overhead equipment to control lighting levels using a gaff

Grips have two main functions. The first is to work closely with the camera department to provide camera support, especially if the camera is mounted to a dolly, crane, or in an unusual position, such as the top of a ladder. Some grips may specialize in operating camera dollies or camera cranes. The second main function of grips is to work closely with the electrical department to create lighting set-ups necessary for a shot under the direction of the Director of Photography.

Essentially these are the same occupation. one uses onlymuscle (Grip) the other usess muscle and a little intelligence.
It doent take much intelligence to pass a high school education. It doesnt take much more intelligence to be a gaffer rather than a grip

Thanks you!