Yern and Jur'ak D'aghar made the Grey Knights eons ago. Eons of perfecting combat styles, Force abilities. Guess what? They're almost as old as the Jedi. You have no chance, old man. None at all.
—Unidentified Grey Knight to Vrekon Shal, moments before the Knight's death
The Grey Knights were a Force-using organization dedicated to the Balance of the galaxy and the Force.
Jur'ak D'aghar—at the time fairly young—decided along with his friend Yern that they would make an organization dedicated to Balance in the Force. After several decades of deliberation on what exactly to call the organization, the two agreed: they were the Grey Knights.
Assassinations and alliances
Centuries take their toll, and Yern eventually died. However, D'aghar still lived, and Yern was able to become a Force Ghost. Assassinating some major players was necessary for Balance, but for the most part the Grey Knights made alliances with key planets and people. D'aghar became the primary field operative and head of the Grey Knights.
A traitor, identified later as Reylon Snim, within the ranks of the Grey Knights took out most of their number before being killed by Ondre Bersson.
Coming out of the shadows
In 10,056 ABY, the Black Knight Crisis forced the Grey Knights out of hiding for the first time. After the conflict, the majority of their number were killed or captured.
After the White Knight Crisis, Jur'ak D'aghar was elected Emperor of the new galaxy-spanning power: the Grey Empire. His enforcers were the Grey Knights, and their role as peacekeepers rather than spies threw many of the older Knights for a loop. However, the Grey Knights continued to remain pure.
Training regimen and rank progression
Rather unorthodox, if you ask me.
The training is separated into five ranks: Youngling, Apprentice, Knight, Teacher, and Master.
"Would you like to fight Sith, travel the galaxy, and get a lightsaber? Cause if you become a Gray Knight, that's exactly what you'll do."
"You had me at fight Sith."
-Jur'ak D'aghar and Ondre Bersson
Not as selective as the Jedi, the Grey Knights' only requirement is Force sensitivity and being around age 4. In cases of extreme Force ability, a child may be recruited at a later age. Such is the case of Ondre Bersson, who was ten when he was recruited. A set number of Younglings were recruited each year, usually fifty.
Starting at age 4 and going to age 12, the Younglings learn basic Force skills and simpler hand-to-hand combat forms. What combat form is decided by the Masters for each child using the Force. Every standard year, the Younglings participate in a tournament, to decide which is the most accomplished of their age group. There is no real prize for winning, and the tournament is all in fun.
After each tournament, the Knights who had been Knights for exactly five years got to choose their apprentices from the oldest Younglings. The order of choice is determined beforehand by the Masters. Thus, the Youngling became an Apprentice, serving his Teacher and being taught in return. The Apprentice must obey his Teacher at all times, and he/she often begins to take on dozens of their Teacher's habits. After the first year—which focuses on the Apprentice's personality, responsibility, and discipline, the Teacher gets to teach the Apprentice whatever skills necessary to do their job, and they go on missions with their Teacher more often.
After five years, the Apprentice becomes a Knight in an unknown ceremony. The new Knight is given their first solo mission a week later. Five years as a Knight grants them the ability to choose an Apprentice for themselves.
The role of Teacher lasts only five years, and ends after the Apprentices are graduated. Each Teacher can become a Teacher only once every ten years for thirty years. After the first time, every ten years later, they get the choice to pick another Apprentice. However, most opt not to.
The only rank that is earned not given, the Masters are the most meritable and mature Grey Knights and every time one dies (no matter the cause) the rest of the Masters choose the new one. There are twenty Masters at any given time, plus the Grand Master, the highest-ranking Grey Knight and the most powerful. The longest-reigning Grand Master? Jur'ak D'aghar, who served in that regard for twenty millennia.