I gave them a reasonably mild prune last spring, and they wont grow back.
Any new growth looks like it burns.
Reads like you have overfertilised your gardenias. Since you write in October then you have had a lot of rain and nature should have cured your problem. If the new growth is still burning on the edges then it is probably too much water. But I don't know what to do. Then water when drier that should be about once a week and see if that helps.