I've thought about it a bit today, and the neatest solution would be to just have Axel be the guy Gillian cheated with, and Gillian take the role of Cherry as the girl Axel cheated with. It would also make for a more original and spicy way for Ian and Lena to meet and start talking to each other.The whole "Gillian left me, I can't get over it" storyline would hit a lot harder if she left him for one of his friends, and both of them were present in the story. You could make Gillian appear in every chapter almost like a spectre haunting Ian, and the male friend could be present in the story as a rival (instead of Robert, lol), maybe culminating by being the final opponent in the MMA tournament, or the book competition, or both.
Then have Gillian hang around because she's part of their friend group. Again, she could just replace Cherry.
Still, whatever the best story solution would be, the general point is that Gillian is massively underutilized in the game. Hot character model and a breakup that supposedly totally wrecked our MC, early tease with Ian looking at her photos and having dreams about her, and then... nothing for like 10 chapters straight. Lena is constantly bumping into Axel, bothered and tempted by him. Gillian should have been constantly opening Ian's wounds the same way.