The greatest element, in Sir Gawain and the Green Knight, which illustrates the atmosphere of luxury is the Christmas celebration in King Arthur's castle. The text offers many textual instances which illustrate luxury.
The feast was alike full fifteen days.
Here, the text reveals that the Christmas celebrations took place over the course of fifteen days. Celebrating with an entire kingdom for fifteen days illustrates luxury.
With all the meat and mirth men could devise.
Here, the people at the celebration had all of the meat they desired. Again, to be able to serve that much meat only suggests a luxuriance.
Then the first course came with crack of trumpets.
Not only were the followers of King Arthur served as much meat as they desired for fifteen days, they were served their food with trumpets playing. Again, an accompaniment like trumpets illustrates the luxury of Arthur's celebration.