Imagine being hung up on Gillian when there are literal sirens around the town. If my Ian was hung up on Gillian, I'd straight away make him a cuck and a loser in every aspect of his life. Sorry, not sorry!
Ian spent many years with her, not something you can't automatically forget even if you want to. His friends comment on how the relationship seemed to be a great one. We also don't know the exact circumstances of the breakup, nor how Gillian sees that and Ian presently.
So currently, her plotline is a decent mystery with tons of potential (even more so if her new boyfriend is Mike).
Contrast it with Lena/Axel, which was shown to be toxic in the prologue and which doesn't have any mysteries or secrets left.
Regarding the cuck route, it gives Ian a plausible motivation/reason to be a cuck, rather just transforming a normal guy to absolute doormat without elaboration/justification.