Well, the way "Grey Jedi" are explained in the lore, is not that they where "neytral" neither dark, nor light, but that they, like Qui Gon would not strictly adhere to Councills decisions - they remain fully lightsiders, but sometimes choose their own path, instead of following directives. So, if you want to be a Grey Jedi, touch luck - you still gotta be light. If you are no longer a lightsider, you become a Fallen Jedi....