His title was being threatened, as a Marquess of the land he must do everything to secure his Title.

This woman, she is a threat to Gus, she wanted to take over his title as the Marquess and become the Marchioness. He saw her as his enemy, or rather his rival

"I've been the Marquess of this land for years, do you really think I'd give it up for someone like you?" *He said with annoyed face"

"Do you even have the experience? No? I thought so." He adds sarcasticly