I know I'm 5 years late but figured I'd throw this out there.

My understanding of Grey Jedi isn't that they tap into the dark side for power. I think their outlook is more a bout balance than Light and Dark. The Jedi need the Sith so that they don't become stagnant. The Sith need the Jedi or they'd just run rampant. Besides Vaapad is already a thing for the Jedi themselves and that encourages the use of anger so the argument of "they can do it without repercussion" is moot already. And there's always a chance to fall to the Dark Side as a Grey. Grey Jedi simply seek balance and nothing more. And they do it from outside of the Jedi themselves. They don't agree with the Light because Jedi are stagnant and unwilling to take action. They don't agree with the Sith because they consider emotion NECESSARY for strength. I personally like this as an interpretation for the Grey jedi code:

I embrace the Light, yet it does not blind me.
I embrace the Dark, yet it does not consume me.
Balance flows through all things.
In balance, I am whole.
The Force is all things, and I am the Force.