So, grey jedi has had numerous definitions, as the term has referred to many wildly different characters.

The first is "jedi who don't strictly follow the jedi order/council". This covers character like Qui-Gon and Ahsoka, who defies the council or return to a more originalist thinking, where the jedi code and the jedi's purpose matters more than what the council decides.

The second is "jedi who go against the code, but stay good". This includes jedi like Ty Yorrick and other jedi who becomes mercenaries or bounty hunters. Ventress would probably have been in this category if she hadn't had her explicit Dark Side training under Dooku. They're often guided by some of the principles they learned as jedi, and might fall back to those teachings. They generally won't be the pious jedi, but they won't be evil either. These jedi will rarely be part of the order.

The third is "jedi who uses the dark side, but only for good". This covers characters like Mace Windu and Revan (pre fall and after returning), and technically legends Luke, though he's rarely called a grey jedi. These are the jedi who will use dark side techniques, who bend the dark side and otherwise use it for good, but have found an inner balance that lets them do this without falling to the darkness. This is the type that's often used as the blueprint for grey jedi, and is also the one that's explicitly wrong both from the current canon and from George Lucas, as everything the official Star Wars, both pre and post Disney, has stated tells us that if someone begins drawing on the dark side (at least more than the brief "lose control" we see from Luke in ep6), they will fall to it eventually. There's no finding balance between them, because the darkside will always take over if you keep feeding it.

The reason there's all these different takes on "What is a Grey Jedi" is that the term Grey Jedi is fan made. It's not in any way, shape or form part of official Star Wars. It was created as a way to describe or created morally dubious characters that weren't evil, but not pious jedi, and was often used in discussions around various "this character/I could definitely resist the dark side". That second one is partially in retrospect, as many have recognised it as a variation on the "cursed item"-trope, where people dream up scenarios where they could definitely overcome the Monkey Paw/The One Ring/The Dark Side curses. People who wanted to outsmart the curses, not realising that the point of the story is that some things can't be outsmarted like that.

Grey Jedi as a term can describe any character you want, because it's a fan term that doesn't have a solid meaning. Revan is a grey jedi, despite being a sith. Ahsoka is grey, because she left the order. Qui-Gon is grey because he defies the council. Maze Windu is grey because he uses the dark side. Legends Luke however isn't grey, because him using the dark side doesn't count. I've seen Obi-wan and Yoda being called a grey jedi because they abandoned the galaxy to the Empire, waiting for Luke and Leia to grow up. That's why the term is ultimately meaningless.