In the beginning they rub each other the wrong way entirely - she's annoying and intrusive and overly moral, so Ianto told her to try not to let being a good person get in the way - he's already learned that in Torchwood sometimes you need to be able to do the difficult, bad things in order to save the day. Gwen doesn't learn that for a very long time, and nor does she mellow out until around when Jack leaves and she's forced to take leadership of the team. :) So this very much follows their friendship in canon, from "Who was your last snog?" "Lisa. You forgot didn't you?" to Sleeper, walking away from the cryo chambers arm-in-arm, to Lost Souls, where Ianto's bike has a bell on it. </3 Thank you!