I take the mindset of the Jedi by their original concept of the ideal of a samurai, its virtues and a lot of zen philosophy.
I take the Sith mindset as the hunger for power of corporations and governments that Lucas tried to criticize in the form of Vader and the Emperor: a person who sold his humanity to a patriarchal authority whose hunger for power is never enough...
But the light and dark side of the force has nothing to do with jedi/sith dogmas: both are ways of dealing with the force, opposite ones, both sides of the same coin, and both refer always to emotional states.
I interpret emotional states as vibrations, as an exponential curve. Low and dense emotions (fear, wrath, hate, anger, vanity, apathy, malice, jealously...) resonate with the dark side, while higher (vibrational speaking) emotions resonate with the light side (compassion, courage, honesty, self-steem, benevolence, love...). Some powers resonate with the dark side and others with the light side. This means that to use a dark side power you have to lower your vibration to use it, and so is easier to begin to be clouded by lower emotions too. Mechanically speaking, using dark side pips to fuel force powers is to "vibrate low". Also, using a power in a low emotional state, you also begin to vibrate "low". And force sensitive beings are more exposed than others to this vibrations.
Is like a glass full of clean water and you begin to put oil in it. Jedi try to keep it clean. Sith not only doesn't care, but the dirtier it is, the better, more fuel for their dark powers.
Gray Jedi...? It remembers me more of a Jedi Sentinel doing the dirty work and, without a strong mind, a Grey Jedi will eventually fall if he abuses his dark powers and the rate that he dirts its water is higher than the rate he cleans it.
Ahsoka Tano? I don't see her as a gray jedi. She labels herself as no Jedi, because beign a Jedi also implies beign part of a structured organitation with an agenda, organitation that betrayed her and with which she feels that not belongs to (like "hey, I believe in God, but I want to know nothing about your Catholic/Protestant/Orthodox/Evangelist Church. I will follow my own way and my own beliefs"). But at her core, she tries to follow the light side. Take notice that she does not justify nor negates her anger when confronted with Vader: she assumes it, she knows herself, and is this knowledge that keeps her water as clean as possible. Is the negation of the anger or submiting to it and begin to project the anger on others and fuel it that leads to the dark side.
I see the Witcher Geralt of Rivia more grey jedi than Ahsoka.