I don't think anyone has a recipe for long term success yet. At least not one that is readily duplicated. I've read a few posts where people have had long term success, but that's the exception rather than the rule.
FWIW, my Diodogorgia sp. also doesn't respond much to phyto. They need something with more protein. Mine will consume whole brine shrimp, chopped up formula one, and prime reef flakes. I haven't tried GP's, but have heard good things about them. I try to feed it a couple times a day when the polyps are out. I've also got it on the receiving end of a 150 gal refuge. In spite of all this I've only seen maybe an inch of new growth since I've had it.
They are beautiful, but a lot of work.
Dan