I could find no information on Grunhill in either Marvel or Norse Mythology. I think it might just be a Norse sounding name the writers came up for that scene without basing it on somebody.
The nearest I could see is Brunilda (Brynhildr, Brunhild, Brünnhilde, Brynhild) who was a Valkyrie in Norse mythology and or course Valkyrie in Marvel comics.
perhaps it was means to be Brunhilda and the spelling/pronunciation got garbled somewhere along the way ending up with that we got on screen.
Another possibility is just Thor showboating and it's just something Volstagg brewed up in his basement and just wanted to hype is up for the silly mortals.