I always thought that Gwen should have understood, given that he'd twice referenced the Doctor when talking to her, and if she'd put the same effort into researching that as she did with Carys in the sex gas episode, then she would have understood why Jack left.
I don't believe Ianto didn't figure it out. He was at Canary Wharf, he would have known as much about the Doctor as any Torchwood employee, and he would have known the Doctor's propensity for abrupt departures. He had to have realised that Jack had to leave quickly, that there was no time for goodbyes.

I will be writing that follow-up piece, not as a Gwen-bash, but as a further insight into Ianto's understanding of Jack's state of mind, which could not have been good at that stage.

Thank you. :)