Sarcasm aside, i don't think any sort of backstory would successfully paint that as an act of goodness. So at best you'll get "she did a Shit Thing... For Reasons". But it'll still be a shit thing.But is she bad though? Sure she cheated on Ian, but we have no clue as to the full story to what had happened.
From what little we were told so far it seems Gillian wanted to do her own thing while Ian was doing his in the new city. She was supposed to join him eventually but found someone else and kind of neglected to mention that and dissolve her ongoing relationship before starting a new one.