Obviously, I'd like to keep the 20. Regardless, the AOO:

(1d20+13)[16] kukri attack vs prone gnoll
(1d20+13)[14] CC 31+
(1d4+6)[8] damage
(1d4+6)[10] bonus damage

not flanking, and prone doesn't appear to remove dex mod from the equation, so I don't think sneak attack applies? If I'm missing something: (4d8)[12] sneak stab