Nothing can replace a Gazelle. Trust me, I have tried. A few months back I wanted to convert all to Trilogy in hopes of making the team. There were two discs that I was having troubles replacing; the Gazelle and the TeeBird. While Trilogy does not have a TeeBird replacement either (I tried the Saint, Escape, and Stag), I can mask its absence through the overlap created by the Truth, Gazelle, and Saint Pro.
However, there is nothing that flies anything remotely like my Star Gazelles. The Gazelle excels at being torqued, especially low to the ground, without flipping, turning, or fading. It is also great at holding singular lines and does not stall out or do anything stupid when given air underneath it. At that power, even my workhorse Truths start to lose some reliability. I can torque-up a Verdict, but it still has a stronger finish than I would like.
It is all about applied-torque. The Gazelle can handle it whereas no other disc I have thrown can. Even the ESP Cyclone has more turn and an S-Curve flight. I suppose there are some MVPs that could handle it, but the rim depth on their fairway drivers always seemed shallow, unlike the very comfortable, grip-friendly Gazelle.
In all, I gave up trying trying to go ALL Trilogy solely because I could not replace this one disc. Everything else is Trilogy, though.