G, J, K, U, W, Y, and Z are all characters that aren't used in Latin (in the case of G, J, U, and W, because they didn't exist; in the case of Y, it is used in Greek words but doesn't represent a sound that's possible in Latin; I'm not sure if Z was used in Greek words or not. K is just a Greek letter -- since it represents the sound written with C in Latin, it is never used at all).