Dear Leia,
I don't remember if Judith Proctor has the story at her site or not with Gareth's credits. But presumably Sheelagh Wells was working on the show and the actor who was supposed to play the character that GT played couldn't for some reason and they were stuck. Sheelah called Gareth and asked him if he would do it as a favor and he got in the car and drove there and did it the next day--or maybe even the same day. (I've forgotten the story since it's been a while.) It's a totally nothing part but Sheelagh knew that Gareth loved Vikings and would do it if he could.
It's too bad they didn't have time to rewrite the part and give him more to say!!
Joyce Bowen