Sefer HaBahir 20

1[They asked,] “And why is there a tail at the bottom of Gimmel?" [Rabbi Amorai] said, "The Gimmel has a head above and is similar to a pipe. What is a pipe? Something that draws [water] from above and empties below. So too, Gimmel draws by way of the head and empties by way of the tail. This is Gimmel.”