Gloria thought about it. Does she know about Marcella's ex? No much. But she knows enough.
He was an asshole who hurt her right to her soul. She watched her heal after the impact he caused on her. That told her everything.
Tony was fidgeting with his hands and Gloria saw that he was holding back himself. She figured out Tony has an idea of the man she was with. She didn't know who Marcella's ex-boyfriend was. She only knows Marcella for three years. But Tony knew her for years. He grew up with her. He would see the changes in her because of her entanglement with him.
Or it was just his masculine urge to want to bash the guy who was loved by his wife in the past?
Whatever the reason was, Gloria didn't want to throw gasoline in the fire.
But she knows that Tony is a powerful guy. Men can lose their minds for a moment out of rage and it can ruin their future. She didn't know if Tony has control over his urges and she didn't want to take a chance—for Marcella's sake.