Assuming Gillian/Mike is true: I wonder if she will always break up with Mike and go for Ian again. Apparantly this is only possible if Lena goes for Mike, but Ivy also stated that she was interested in Mike. Now, this could've been a casual filler comment without any meaning but to react to Lena's partners (when Ivy said that, she was also commenting on Lena's other partners). But if it wasn't casual, Ivy could seduce Mike and Gillian would always be dumped.
Otherwise, Gillian entering Ians life again is irrelevant since she is almost always content with Mike.