But...I think he has good reason to do that.
I mean, all his life the guy has been told he was born wrong; First he's raised as a daughter because he looks feminine and likes the moon. Then he has the whole snake feet thing going on and I really doubt that helped.
I mean, I was Harvesting in Anor Londo and there were no statues at all of him. Even in the massive Cathedral where you fight Ornstein and Smoug it only has Gwyn, the empty plinth where the First Born would be and Gwynevere. Kinda rude and insensitive if you ask me; it's like Gwyn was ashamed of his frail son and didn't want anyone else to really interact with him. The game even describes him as "repulsive" at one point. That's nearly as harsh as the treatment of Priscilla.
Gwyndolin is also the only God to stay in Anor Londo to try and keep the place going; everyone else just leaves. Even Gwynevere buggers off with her husband and the First Born is AWOL. Everyone else is too selfish to stay and try and keep things in order due to their fear of the dark so they just run away; Gwyndolin on the other hand protects his fathers tomb and has his illusions keep the city safe.
So it makes sense he would have the illusion of Gwynevere greet the player; she's the beautiful daughter of Gwyn, the Princess of Sunlight and basically the most kind, compassionate person ever and is attractive while Gwyndolin is the freakish son/daughter snake thing that no one talks about. So I figure he would guess the Undead would likewise be repulsed by him, hence him making the Gwynevere illusion.
I feel so bad for the guy. He seems pretty damn happy when you come visit him if you are a Blade of the Darkmoon. It's probably lonely as hell in that Tomb. I only kill him on the my Darkwraith/Path of the Dragon playthroughs. Otherwise I leave him alone.
What about you guys? Do you like/hate/kill Gwyndolin all the time?