I confirm Sushi is female. Zak does refer to his pet as “she” or “her” in one of his lines AFAIR. Plus I always interpreted it as a play of words with Suzy.
Myself, I am -to my wife’s content- very much male though. I am just a huge Zak fan who took it as an avatar that would be recognisable to the right people and a mystery to the rest of the world (hell, my mail address even includes the “in a fishbowl” part!).
I guess most people here on this forum are male. Except perhaps @PiecesOfKate (which is another nice play of words with the annoying in-jokes enabled)