Sorry, I missed this thread.
I do not know of a way to regenerate GFO in the sense of removing phosphate without dissolving the substrate itself. Low ph is likelythe wayt to go, but I do not know any specific of whether there is any pH window where you can dissolve the phospahte and not the whole GFO.
Heating it to high temperatures (500 deg C perhaps) will drive off orgnics, and that might help restore some phosphate binding ability that was taken up by them.