The Game of the Green Knight

In SIr Gawain and Green Knight, the Green Knight is a rather enigmatic figure. He walks in and challenges King Arthur to a game and that game is to cut off their heads. Sir Gawain would cut the Green Knights head off first and a year later later King Arthur would come to the Green Knight for his head to be cut off. Everybody thought this was a joke, but it was quickly obvious that the Green Knight was serious. Arthur was about to accept but Gawain accepted in his place and he decapitated the Green Knight. The Green Knight picks up his head and just leaves casually.

Why would the Green Knight treat his life as a game like this? no matter what Gawain did to the Green Knight, he would not die. if death is not something to fear it will eventually become a game. If I was immortal, I would treat it like a game. Like trying to die in the stupidest way possible. The Green Knight must have a similar mindset.