Casy/Cassie is the average King Gizzard name, let me explain:

If you lay out all six names, we can observe a few things:

1) The most common ending is Y/EY/IE (which all sound the same)

2) There are unique letters in each name (which should be removed to find the average name):
Cavs - V
Stu - T
Joey - J
Amby - MB
Cookie - I
Lukey/Lucas - L

3) The most common beginning sound is "C," from Cavs and Cookie, and the same sound also appears in Lukey.

4) Two vowels sounds are repeated, "A" in Amby and Cavs, and the "u" in Stu and Lukey. The "oo" and the "oe" both sound slightly different. Since "Cusey" is not a name, I picked "Cassie." Pronounced "Cass-E" not "Case-E."

5) The average name length is 4.333 letters, but varies between 3 and 6. Ideally the name would be spelled "Casy," but most people would spell that pronunciation as "Cassie," so I have included both.

NOTE: This is based on the use of their nicknames. If their given names are used, this calculation might look very different (Michael, Stewart, Joseph, Ambrose, ???, and Lucas). I did not do that calculation.