The issue with g/f with *some* dogs is that the substitute for the grains are legumes (peas). Peas, it seems, interferes with taurine absorption, and that's the suspect here in cardiomyopathy. Affected dogs also seem to be eating a lower quality kibble that is not meat (and red meat) rich.
I feed (and have fed) g/f exclusively for at least 15 yrs w/o trouble, I think the grains and cereals are more problematic than peas, especially with high performance V's. I feed a meat based, high protein kibble (Nature's Variety Instinct), rotate among the meat proteins, and add fresh meat of my own. You can certainly rotate a high quality grain feed that is also meat based if you are concerned, but if a red meat meal is one of the first two ingredients, you're probably fine.
In your case, b/c the vet recommended g/f for medical reasons and your V is responding well to it, I personally wouldn't panic, just make sure it's a high quality meat based kibble, feel free to rotate. The Omegas you add are not related to this nor will backstop this type of cardiomyopathy, btw.