Jack felt the warmth of Gwen's approval for his compassion toward Ianto. In reality, he was making plans.
What Ianto had feigned to save Lisa would become real and what Jack might once have felt would become a lie. He could adopt a mask of monogamy or play at domesticity if necessary. In the end, Ianto would be absolutely convinced of Jack's love.
Gwen might make an excellent instrument for the final betrayal, but the who was relatively unimportant, only the end result; Ianto's suffering when the truth was revealed.
No one conned Jack Harkness and got away with it.