Well if we're being serious, I don't think anything actually explicitly states the White-Luck Warrior belongs to Yatwer. This White-Luck Warrior does, though. There's no reason to think Gilgaol couldn't make one, if he wanted, I suppose.
As to what Gilgaol has to do with Yatwer and her pawns...allegedly, Gilgaol is Yatwer's brother. And Birth and War are the two most popular of the Gods.