The Gooper is the opposite of the pooper, which is to say that it is another word for a girl's pussy. The term is used almost exclusively when describing the manual method of action for the "Shocker", which is: "Two in the gooper, one in the pooper". Which refers to putting the index and middle finger in the "gooper" and the pinky finger in the pooper while the ring finger is folded down and the thumb is extended out.
I had her all riled up and wet from a shocker after throwing a couple fingers in her gooper, and she didn't mind the pinky in the pooper! "Two in the gooper, one in the pooper!"