This has been a big complaint of mine (and a lot of other people) for a long time. He was nowhere to be found the last time Gwen was getting banished, too -- and he wasn't a knight, then, so he doesn't even have political cowardice as an excuse. They really haven't interacted at all this season, other than when Elyan was wounded in 4.08.

Like I said to someone else below, I think it's a symptom of a greater writing problem, which is that they have never treated Gwen like a primary character whose thoughts, feelings, and reactions need to be shown on screen.