I wouldn't call Gay an high-end defender. He is just good and this forces O'neale to defend guards more which has been a problem. May be Royce would do better with Gay around but I am not sure about that.The Bulls are still 19.5 under the cap.
The Jazz can trade Bojan + Scrub for Lauri. That dumps some salary and gives the Jazz a backup 5.
Gobert
Markannen
Gay
Oneale
Mitchell
That's some crazy length and 3 high end defenders (Gobert, Gay,Oneale).