Luke's comment in the teaser trailer for TLJ means nothing at this point. There's no context to know what he means. Not to mention there's always a chance it may not even be in the movie at all if you consider what was absent in TFA and R1 from their respective trailers.
I don't see how a concept of "grey jedi" could fit into the current star wars canon. Not following rules is not the same as following the dark side. Neither is being aggressive the same as abandoning the light side. Do you think Qui-Gon is represented as a dark side user? What about the bendu? Do you think he is grey or just ambiguous and indifferent?
There is no grey side of the force in star wars. We are told that the dark side is about following ones selfish passions and lust for power and control, where as following the light side is about selflessness and compassion and submission and love. I don't see how you could logically embrace both of these at the same time. It kinda makes for a ridiculous hypocritical character.