Throughout the Seasons
Fiona decides to go talk and end things with Gus, after Debbie accuses her of "ghosting" him. She shows up at one of his performances and he asks her stay because he wrote a song for her.
Fiona decides to reach out to Gus in The F Word. At first, Gus pretends not to be holding any resentment towards Fiona. However, he then proceeds to sing a song on stage, in which he gravely insults her, revealing that it was all just an act.
As Fiona tries to buy the Gallagher home back, her adviser sees a problem in her still being married to Gus. Fiona is asked to get Gus to sign a sheet in which he swears not to make any claims to his entitled share to the house. However, Gus, seeing no reason to do Fiona a favor, destroys the form right in front of her.
He returns in "A Yut To One's Own" to seek divorce from Fiona. He promises a quiet divorce in return for having his grandmother's engagement ring back. Fiona pretends to agree, but is unable to repurchase the ring she had previously pawned. When Gus learns that Fiona's lawyer is (against her will) making inquiries about Gus' possessions, he is furious, even more so when Fiona admits to having put his ring in pawn. At the divorce suit, Gus' lawyer discloses their intention to charge Fiona $15,000 for the ring.
This makes Fiona reluctantly agree to file for Gus' possessions. When Sean shows up with Gus' ring, he is then forced to watch him propose to his soon to-be ex-wife.