Although Sir Gawain and there Green Knight possesses very obvious Christian elements (Gawain's shield and his desire to pray/worship), the text contains multiple hidden pagan elements as well.
Many different aspects of Paganism are tied to the waiting period of a year and a day (the time period for which Gawain must meet the Green Knight to receive his own blow). Historically, one must wait for a year and a day in Wiccan religions to move upward in initiation rites and name changes. Also, feudal laws named slaves freed if the were able to stay away from their master for a year and a day. Also, a year and a day is considered the perfect amount of time in regards to magic and magical events.
The Beheading Game (or Ba') is pagan in nature. For Pagans, Ba' represented both death and rebirth. In regards to Sir Gawain and the Green Knight, this game helps to insure Gawain's death as one who feels secondary to his rebirth as a hero.