Ya, third one is my least favorite for many of the same reasons (a lot of that is the fault of the story basically taking liberally from Holy Blood, Holy Grail conspiracy).
Actually as far as story I personally like the second the best. Followed by the first.
Sad thing is we'll never get part 4... Which probably would have seen GK grow out of his Douche baggery and 'grow up', and stop being the ove-raged 'player'.
There is a cut scene in the game script where you learn that Grace is pregnant with Gabriel's child. That would I think certainly lead him to settle down, and apologize for his chauvinistic behavior. Because deep down I think he really does care for her, even if he's lousy at showing it.
Also if you weren't paying attention Gabriel's douchebaggery appears in GK2's story as well. In as much that he ran off on his dangerous mission without telling Grace anything about it, and trying to keep her out of it as well. Which turned out to put him at extreme risk considering he didn't even bother to do his own research before leaving. He turns out to be fairly rude to her when she shows up uninvited. That's not to say that Grace is also a bit jealous harpy and mistreats Gerde, and Gerde is tad suspicious of her involvement since she was not invited to come in the first place.
They're both fairly selfish and stupid as characters; albeit they jump to doing things without telling each other. In the third game Gabe learns about the temple only minutes before he has to go there, not even caring enough to ask Grace in-between. I tried asking her in the small instance before going to meet Prince James and his answer was "meh, I don't feel like talking to her right now".
As for his douchebaggery in the second game, it isn't as prevalent due to the fact that when playing as him you spend almost all of the game fraternizing with the hunting club uber-douchebags.
Anyway, good adventure games, I can understand why they really made an impact when they were released.