So I have an argument for Gray Jedi being canon only so much in as they exist. In 2010 the book The Jedi Path was published by LucasFilm LTD, as a reference book. All reference books were stated to be G-Canon. Then in 2014 it was all washed away and restructured. Only the movies the Clone Wars Animated series with Dave Filoni and a few other exceptions remained canon. Then in The Last Jedi we see the actual supposed book The Jedi Path on Luke's shelf.
Rian Johnson then showed the book on his twitter. As well as the section that talks about the advanced force technique Doppelganger. Also known as Similfuturus to show that some of the abilities shown were just random made up abilities. But instead were techniques that had a place in the Star Wars universe.
Carrying on that thread when rewatching The Clone Wars after having read The Jedi Path I saw more significance in the scene were Jocasta Nu and Obi-Wan are talking about Dooku. They speak about how he resigned from his position in The Jedi Order. Stating that "I believe he lost faith in the Republic" Jocasta Nu. If you look before it pans in and you see that Obi-Wan is standing in front of Dooku's bust there other busts in the library. Now on page 151 of The Jedi Path under the section Threats To Jedi Teachings By Restelly Quist, Chief Librarian they speak of The Lost. Jedi Masters whom resigned from The Jedi Order due to philosophical reasons. In the margins Anakin's note refers to them as The Lost Twenty, Dooku being the latest. Back in the library in The Clone Wars scene, I was able to count twenty busts. Assuming that Jedi Masters whom resigned would hold a special place in The Orders history and therefore would receive special acknowledgement. I believe these busts to be the busts of The Lost Twenty.
If this is the case then I propose that that entire page under the section Threats To The Jedi Teachings in The Jedi Path is therefore canon. On that page the paragraph before The Lost Twenty's paragraph it states.
"The so-called gray Jedi have been with us since the beginning. Although they do not break with the Jedo orthodoxy concerning the dark side, they bristle when asked to take orders from the Council. Gray Jedi make compromises, cut corners and hide their actions from scrutiny, all under the assumption that their experience makes them authorities on policy. They are Mavericks who are difficult to control, but can be valued members of the Order after they have been persuaded to follow the established heirarchy."
Obi-Wan's note further states "Some call Master Qui-Gon a gray Jedi, and I confess I can see why they do. He's going to get me into trouble".
This is my theory for gray Jedi being canon. Again only in so much as they exist not that they are what we fans have made them into. Furthermore this would also make Potentium canon as well since another paragraph on that page talks about the Potentium philosophy. Though it refers to them as relativist instead of Potentium.
This is my first post here and are my thoughts after diving headfirst down the Star Wars rabbit hole in preparation for season 2 of The Mandolarian. I posted a less flushed out theory on the Grey Jedi Code wiki and someone suggested I post the theory here if I wanted more responses. So here it is would love to here some feedback on this theory, for or against feel free. I'm only throwing this theory out there to hear what others think. And again my first post here in the wookiepedia discussion area so please be gentle.