1. Gippal became cocky and confident after he lost the eye -- he figures that he's used up his share of bad luck, and nothing else that can happen to him will ever be worse.
2. He was among the first Al Bhed to meet with Crusaders who wanted to experiment with machina. Getting to know them was what gave him the idea to enlist.
3. He's not a slut, exactly, but he genuninely likes most people, and he's a physical guy, and sometimes that leads to bed. Because he's careful never to create false expectations, almost no one ever holds it against him, and his life is full of former lovers who are now friends.