I wouldn't label him at all actually. According to the novels, Galen Marek died in the first game. And I don't mean the climactic ending, but when Vader shoves him out of the window to show The Emperor that he didn't care about him. At this point Vader already had a bunch of clones, the Starkiller you play in the other half of the first game and the entirety of Force Unleashed II (aka, the Starkiller that fell in love with Juno). So technically… he has never really been part of either faction. But he isn't about balance either, since he mostly uses Dark Side powers.
Think he's a little like The Exile from KOTOR II. My Exile, Jocelyn Maura, despised the Force and all it's uses. She despised the Jedi ánd the Sith. She uses the Force as a tool for good, but doesn't accept it to control her actions. I think Starkiller is a bit like her, a neutral character. But not really a Gray Jedi either…