Hmm. We might understand it a little better once we see the Movie once it comes out, ya know.
So let me try and guess. Basically a Grey Jedi is a Person who are Force Wielders who don't believe in both the Light and Dark Side. However, they trained themselves or by someone so they can embrace all of it from both Sides, and they are considered the Most Powerful and Strongest Wielders and are even free from the Force.
So basically they are like Silver when he uses his Psychokinesis, but with a more stronger version of the Force, right?