'grey jedi' implies having your cake and eating it too between very contradictory paths.
As I note here, the idea of an in between is a modern thought that goes against how the dichotomy originally worked. There's another exception I dread tossing in which is Revan from KOTOR but I don't think it substantially changes what I say, meaningful gains can be explained for him with sheer potency and not because he separately harnessed two different power sources in a 1+1=2 equation. At very best the answer is only 1, 1 being your maximum potential with space wizard genes, and what you do is hone your abilities until you reach that, regardless if it is through light or dark usage.
I would say that the sides, being maximally potent only if you entirely commit to them (even the full extent of the dark side is considerable effort as we see), that you will never get more than either by sitting on the fence. You can only hope to tread the line very carefully, which is a valid utilization of the force if you can make it work but is not a galactic loophole. A writer in the future or even now could disagree but there's enough from the top authorities + room to wiggle that I'm comfortable in this spot.
These ways of the force are not equivalent to what Jedi and Sith are. The Jedi base their philosophies on the light, but they are not the only way to do it. Sequels aside, I think a moral of the original trilogy (complimented by the prequels) is that the Jedi had lost the way. They were deluded, an opening exploited by fully committed Sith. Sith reach their maximum potential in the dark side regardless of surrounding damage it causes, or their arrogance creating a cosmic 'oh yeah, watch this' in their moment of triumph. They're the oldest dark side organization we know of but there will inevitably be new users in the galaxy who start the journey themselves. They might even be called Sith out of habit but then that's just a name tossed onto a deeper process.
A Grey Jedi is quite arguably more arrogant than both, believing they can harness power based on selflessness and on selfishness. Force power is carried with intent; it's not divided in skill trees but even if it was you'd have the same number of 'points' to spend on abilities and so would be spreading yourself out both ways. Force powers are pretty general in the first place so I don't think there's much appeal in trying to 'unlock' both, and the greatest specialty actions come with your full commitment regardless. Power is a means, its use defines if it is light or dark. If you choose to build or to destroy.
You'd be right if you're suspecting I am not a fan of 'grey jedi'. In force terms they're unremarkable, they can be skilled users but their impact on the galaxy is apathy.