People marry too early and regret it. I don't think Gil is a bad person, he just got bored with his marriage and needed that dopamine rush when we do fall in love. Just a new challenge. It sounded like he really loved her, but had too much to lose with his marriage. He probably has a mortgage, cars, children and all of the complexities marriage has to offer; he has to let go too. Gil is hurting also.
At least Zy didn't get obsessive and went full on 'Basic Instant' on Gil. She kept her cool and learned a life lesson. I think this will make Zy a stronger woman.
Good post.
RE: a mistress love