I don't get why some people are upset with Gillian's return. "Drama for drama's sake" Drama is the point. There are a lot of scenes here that progressed into sex quickly. Would you listen to someone complaining, "Porn for porn's sake"? Unlike other games, ORS actually has good character development and drama. Ian is already recovering, and depending on your choices, could be starting to get into a committed relationship. This will lead to better story arcs. Instead of your typical, "Heartbroken person lost trust but found the right person to love again" trope (more common with female protagonist finding the right man in romance stories), this could lead to more heartaches and send Ian spiralling back, or make proud proclamations of "I have moved on and I am happy" to the person who broke Ian's trust. This could also lead to wild plots like Ian's voyeurism that was explored with Allison or Lena's Stalkfap. Not having Gillian return would be a missed opportunity for the story, especially since this is an interactive fiction where you can choose decisions for characters, and not a novel or TV show.
I will say this though, I wish Gillian's return wasn't so early. There are a lot of subplots where their conflicts are increasing and becoming more tense. Having yet another major conflict introduced so quickly can be disorienting. I wish Ian's relationships were deeper and progressed further before Gillian returned. This way, the "will he or won't he go back" question is much harder to answer.