I'm so sorry to hear that Gatsby is still having problems. Sometimes diet alone just doesn't help. He may have Feline Idiopathic Cystitis. Several years ago my Sunny had this and even though his bladder was empty he would keep jumping in and out of the litter boxes. He was given antibiotics at first which didn't work and then later he was given a muscle relaxant that's used for people and this did the trick.

I kept on trying to think what may have caused this and it turns out that the collar that I made him wear was stressing him out so much that it caused his bladder to become inflamed. So now he's the only one of my cats that doesn't wear a collar. Luckily he hasn't had another episode since. He does eat both canned and dry Innova Evo which may also help him. Here's a good link about cat's with FLUTD: http://www.vet.cornell.edu/fhc/brochures/urinary.html. Lot's of prayers and positive thoughts are being sent Gatsby's way. Please take care.