I feel like that people who ties Gillian to only NTR shit are missing a lot of opportunities. Firstly, we never got background story why she cheated on mc in the first place. When they finally talked with each other, Gillian seem guilty as fuck; she looked like someone who regretted her decision.
Secondly, if she is truly Mike's girlfriend, she could be cucked by Lena and it would be interesting to see her reaction. In another words, she would get the taste of her own medicine lmao. Here we have a lot of interesting scenarios.
If Ian is in relationship with Lena, he gets cucked by Lena and Mike as well. He with Gillian would draw closer sharing the same grief what eventually leads to the restoration of their relationship.
In another case, Gillian develops her own cuckqueen fetish and starts to lose it, or maybe gets vengeful at Lena and starts to sabotage her.
So far, we know for sure that Eva would make netorare route with Gillian for Ian, however, I hope it would not be limited only to it.