Throughout the Seasons
Fiona meets Gus at a bar when she came to see his friend (who unbeknownst to her was in a relationship). Fiona pretends to be with Gus and kisses him which he seems to not mind. At the end of the night they hit off and over the next week enter a passionate relationship. They end up getting married and all seems well until Fiona's ex-boyfriend Jimmy shows up to win Fiona back. Fiona ends up starting an affair with Jimmy which she confesses to Gus. From that point their marriage begins to deteriorate Gus goes on tour with his band and while they have relations when he comes back Fiona realizes she has feelings for her boss/friend Sean and decides to start a relationship with him while she's still married to Gus.
Fiona decides to reach out to Gus in The F Word after Debbie accuses her of "ghosting" him. 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 Yurt Of 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.