Anyone care to speculate what the whole Gillian thing is about? I'm kind of curious.
As you said, we can only speculate.
Gillian did invite Ian for a coffee and added him on instagram, with her account having no pictures of her boyfriend. Not a subtle hint for him that she may be available, just reaching out for him for pure friendship would be a wasted plotline. If the boyfriend is Mike, then he is cheating her. If not with Lena, at the very least with Ivy. So that could be an additional motive that she is already considering to get back to Ian.
She couldn't have control over getting a job in the same place as Ian, this is a coincidence that fits the narrative of the Red String, with people surrounding Ian and Lena coming together by fate. Another argument that she can play a larger role in the game, especially if she is with Mike, who is a character from Lenas side, while she is one of Ians.
As funny as it would be to affirm cuck Ians nightmares of her fucking others in front of him, its questionable if she is that wild in reality and if it isn't just his insecurity nurturing his cuck fetish, with Lena being the only woman who can take full advantage of it. Gillian seems authentic enough that she is sorry and she was with Ian for years, so we can also rule out that she doesn't feel anything for him. Else they wouldn't be together for years in a happy relationship (which was pointed out by others).
Still, Ian can react very different towards her, so there may be multiple routes. Perhaps one where he is clingy and she realizes it and can take advantage of it, maybe one where he is more confident and they treat each other as equals. Or chad Ian getting revenge on her. The most likeliest is a reconciliation in which we get both characters perspective on the breakup. Because we were never told what exactly happened either from Gillians PoV and not even Ian's PoV, so obviously this plot thread can be adressed sooner or later, instead of being a mystery forever.