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I would use Jedi as the base class, with some talents from the Jedi Sentinel talent tree. As for prestige class, I would use Jedi Knight and look at the Jedi Shadow talent tree found in Knights of the Old Republic page 39. In particular, Taint of the Dark Side talent reflects how a Gray Jedi walks the line between light side and dark.

You're asking for the SAGA Edition?  Even if you're looking at an older edition the answer is pretty much the same.

Yes, there is a class/PrC for the Grey Jedi or Revanite.  They are whatever classes happen to have the features you are looking for.  Like Richterbelmont has already stated you'll most likely build the character just like you would any other Jedi character; most likely this means levels in the Jedi base class and eventually levels in the Jedi Knight PrC.  If there are other things you think the character needs I suggest taking a look around at the other classes and seeing what you find there.

With SWSE you should remember that names do NOT need to equal concepts.  The Jedi base class may represent a "Jedi" but it can also represent a number of other things; some character may even multiclass into it for something yet still have NOTHING to do with what the "Jedi" stand for and are.  When it comes to feats and talents you should just look at the game mechanics involved with them and if it fits your character concept just refluff (describe) it to match your vision for the character.

At first I was worried that this would be another redundant PrC but thanks for asking how to make the concept work without resorting to houserule/homebrew matterial.  For what it's worth WotC didn't do a good job of that because it would be VERY easy to combine the Jedi Knight, Sith Apprentice, and Imperial Knight (LECG) PrCs into a single "Force Warrior" class; entry requirements vary slightly but the class features (HD, BAB, ability progression) are identical and there is a LOT of overlap when it comes to available talent trees to choose from.