That is all so vague and wide fetched. Look, as far as I know Ian doesn't know with whom Gillian cheated. So let's talk interesting. What if it was Jeremy? Or Axel? Or Wade? Or, for that matter, Cherry or Alisonouise? Hell, even Robert would be better because Ian already knows and despises him.
As I mentioned, I still doubt that Gillian is Mike's girlfriend, but the only far fetched thing I see is that connection. The rest are just possibilities that open up if it turns out to be the case. I'd prefer if it isn't, because I think it would overdo all the coincidental connections, but I still think it would be interesting because of how it could be used in the existing storylines.
It was also striking how Mike became a more central character once Ian and Lena's potential relationship was established (unlike Robert, who more or less disappeared)—she could cheat with him two or three times in chapter 10, and that musical collaboration was also set up. I take that as a sign that he'll either way be one of the biggest "threats" to Ian in a relationship with Lena.
If Mike is the one Gillian cheated with and is in a relationship with, it's likely Ian finds that out now. Add in Lena's connection to him—Ian could already have seen them together at the end of Ivy's birthday party—and I think Mike would fit very well as a vague threat that Ian doesn't know if he should really be worried about (because history shows he should) or if he's just being paranoid. The guys circling around Lena, both in person and on Stalkfap, generally cause Ian that kind of concern. And at least there's a few details in the story that
might hint about Mike being the one Gillian cheated with. With any other it would be completely out of the blue, as far as I can tell.
Anyway, I think we can agree to disagree about how interesting it would be if Gillian is Mike's girlfriend, and more or less agree that it's still far from certain. And regardless of Mike's role in it, I assume Gillian's presence can turn out in three main ways for Ian (simplified, of course). He can end up getting back together with her (edit: or it could just be sex); he can use the occasion to finally get closure and fully move on; or it can bring everything back and pull him back to step one, almost as miserable as he was right after the break-up.
The earlier choice to continue watching her pictures or not seemed to be related to the last two alternatives. And the fact that Gillian shows up at the exact time Lena is about to leave town for a few days, could fit with the first steps of the first alternative. Now they've bumped into each other, I assume they'll get a chance to meet and talk things through in the coming days, with Lena temporarily left on the sidelines. Which she in turn can end up worrying about too, with or without reason. Independent of Mike I think there's material for some interesting conflicts there. (Of course all of this will play out differently if Ian and Lena aren't in a relationship, and create more or less complicated conflicts. But luckily I don't have to do as Eva and account for every possible path.)