Trapped by a mob in a tower room without his own arms and armor (of which he had been relieved when arriving as a guest), Gawaine accepted such arms as the King of Escavalon’s sister could provide him on short notice. When she could not find him a shield, Gawaine seized a chessboard, spilling its chessmen to the floor, and assured his hostess that it would serve him as well as he could wish.
The ivory pieces were ten times larger than most chessmen, and shortly did service as missiles for the lady to hurl down upon the mob; the board must have been proportionate in size. Thus, heaving it up and tilting its chessmen off sounds like an example of Gawaine’s strength. It also sounds, to me, like a bit of tall-tale humor.