She keeps apologizing to Ian over it, so it's not like she isn't sorry or anything. At the same time since she chose that guy over Ian in the first place and they broke up over it, it'd be pretty silly for her to abandon him just because her now-ex is sore about it.Surprised anyone wants a “vanilla” Gillian route given she already cucked Ian and doesn’t seem particularly remorseful about it even still being with the guy she cheated on him with.
As for vanilla route -- people grow up or experience things and can change their views. That includes who they want to be with.