Hi, welcome to BYC!
Sorry for your gals discomfort/trouble.
I know too much calcium (more than 3%) fed long term to birds NOT in lay CAN cause gout and kidney failure. That include ladies who are molting, roosters, broody's, and juveniles.
Also, Sussex is a dual purpose breed and will do better with about 18-20% protein in their feed than the 16% that is most Layer. Too much protein could also cause a type of gout that is different, but that would be about 30% plus. It may be more prevalent in birds who are predisposed to it. Some people who feed layer the whole life will never experience it because their birds will pass before that.
IMO, the best thing you can do is switch to a flock raiser or grower with only about 1% calcium and provide oyster shell on the side for the ladies who are still laying. Reversal, I don't know... but yes I do think gout can be aggravated or helped with diet and see it in humans all the time. My birds get Purina 20% Flock Raiser (all the labels mean nothing it's about nutrient analysis), their whole life start to finish with OS on the side for the layers. It works for me because I always have varied ages and genders in my flock.
I would also ensure that her roost isn't too high and provide a ramp if needed, though it might require a little teaching how to use if they aren't familiar.
My bottom line is 18-20% protein and 1% ish calcium with oyster shell on the side. Expect it to take a little while to see a difference IF it is going to work/help. Maybe a few weeks or so.
Maybe an occasional raw and unfiltered ACV added to the water wouldn't hurt, for the probiotics. Curious, did you have the gout diagnosed by a vet?
Speckled Sussex are one of the prettiest birds!
Hope this helps and she feels better.