The grain free dog food itself doesn't cause the heart disease.
But a dog on a grain free dog food diet can have a much higher chance of developing heart disease because having grain in the dogs diet leads to a healthier heart and a healthier dog.
So if you feed your dog strictly grain free dog food then the dog might get heart disease due to the lack of grain in the dogs diet.
But the grain free dog food is not the cause of the heart disease.
If you feed your dog grain free dog food you need to add some grain based dog food to the dogs diet to keep the dogs heart healthy.
As long as you incorporate grain into your dogs diet along with feeding the dog some grain free dog food the dog should not get heart disease.