This in regards to the book Star Wars, Aftermath, Empires End:
The Gray Force - Canonized
"...When the final surviving pilgrim gets to the cave where the crystals belong, he gets trapped there. This devotee of the Church of the Force, on what he considers a holy mission, knows that he's now entered his final moments. He listens to the holodisk again, the preaching of the Church. They talk about the lightsaber, and the Kyber crystals, and those that were taken to power the Death Stars (the "legacy of Galen Erso, the legacy of Orson Krennic, of Tarkin and the Sith," incidentally).
As Addar, the young pilgrim, resigns himself to his fate, it says, "where he will join soon with the living Force, all hail the light, the dark, and the gray."
Yes, the gray. This follows a lot of "balance" talk over the course of the last couple of years in Star Wars, from Rebels to Rogue One, from novels to comics. The idea of balance has reigned supreme for the last year, and now there's even an in-universe, canon character recognizing that the Force is not just two sides, but there's a middle, an intersection where light and dark can meet. It's not unlike the message of the Bendu (Star Wars Rebels, also canon), but this is the first time that "Gray" has been used to describe it."