Having read through a lot of the comments, I thought I might put in my two cents worth.
First, and foremost, a lot of the issues people mention are related to "Jedi with Force Lightning" as the main motivator. But I don't honestly think that has anythIing to do with the interest in the Gray Jedi. Justifications for naming Qui-Gon or other canon characters as being "Gray" is more about their being more flexible about how to be Jedi than the code is...a code that has been changed at least once in universe, and is shown in the prequels to be nearly impossible for its adherents to truly embody. Much of the serious discussion seems to be much more about a way to serve the Force that isn't as inflexible as the Jedi Oath. You can call on the Dark Side for the very same things one can call on the Light Side for...it's all about the emotions and decisions behind the use of power.
Yes, Star Wars is rooted in duality. But it's also never shied away from showing that there are plenty of shades of grey in the universe, either. The Prequels, which sprang from Lucas's vision, demonstrate all of that very clearly...especially the ineffectual bureaucratic nature of the Jedi Order. The Jedi Council might consider a Jedi who doesn't keep in step with the Code to be Gray, even if they never use Harm or Unleash or anything like that.
I think this game system is actually really well suited to handle people who live in between the Light and Dark sides, and who might strive for the Light but still fall into Darkness. At the end of the day, dismissing it as video game/power gaming is, to me, more an indictment of the writer than the Grey Jedi concept.
Finally, there's a lot of complaint saying that this sort of change would make the Jedi Order to date seem like chumps, which I think is overly precious and cynical. In history, people thought one thing only to have it turned on its ear years/decades/centuries later. But more importantly, a lot of the complaints are based on extrapolation of a movie that hasn't even been released yet. Grey Jedi are simply an idea floating around...which means it firmly exists in the "Take it or leave it" category. Getting bent out of shape over it is a choice, and one that many people are taking a bit too personally.
For the record, I'm undecided on the concept. It's interesting from a storytelling perspective, but undeveloped in relation to much of what exists. Does that make in incompatible? No. All of this is made up, but its entrenched by decades of fandom. A lot of that is very broadly defined, save for when people choose to adhere to EU material...which is about as valid as the Grey Jedi themselves.
Anyhow, that's my two cents in all it's rambling glory.