I'm glad someone else can see the reaction. I thought it was just me who thought a mirthless grin wasn't exactly what I would have expected.
If you play devil's advocate in a discussion or debate, you express an opinion which you may not agree with but which is very different to what other people have been saying, in order to make the argument more interesting.
https://www.collinsdictionary.com/dictionary/english/devils-advocate
In my opinion Gerry wasn't expecting Mr Paxman to express anything but wholehearted support for him and his wife against those nasty journalists.
Paxman is a journalist perhaps naughtier that most and with a fearsome reputation to maintain. I do not believe that anyone going into an interview with him would expect an easy ride and in my experience few get one.
I do not believe I have ever witnessed Paxman conducting a more sympathetic interview with anyone else than he did with Gerry McCann.
If as I believe it must have been by the questions asked the Leveson Inquiry was in full swing at the time, so one could have expected that Paxman might have gone in there with all guns blazing. Instead he conducted, in my opinion, an interview which was sensitive to the feelings of the parent of a missing child. I think the interview was constructed to be conducted in a way appropriate to reflect the tone of the Leveson testimony that Kate and Gerry had given about their treatment at the hands of the press.
Gerry smiled for a brief few seconds as he listened to a question. So what?