Up to the point that De Troyes' tale was composed, Gawain does not play a game of chess. This game occurs in the continuations to the De Troyes' Perceval. However, attacked at Escavalon once the town realizes that Gawain is the knight who killed their Lord, Gawain and his lover begin to use chess-pieces as weapons:

[S]he hurried to fetch his armour, for she was not feeling at all safe, When she had armed him fully both she and my lord Gawain were less afraid excpet that as luck would have it there was no shield to be found. So my lord Gawain made a shield from a chess-board and said: 'Friend, I don't want you to look for any other shield for me.'

Then he overturned the chessboard, which had ivory pieces, ten times heavier than other pieces and of the hardest bone. Henceforth, whatever might happen , he felt he could defend the door way and entry to the tower [....]

[... T]hey [the mob] splintered the door by hammering it with axes, finally splintering it in two. But Gawain the doorkeeper held out strongly from within [....] The damsel took the chess pieces that were lying on the stone floor and flung them furiously at the mob. She tore at her hair and flailed about and swore in her wrath that she would have them all destroyed, if she could, before she died. The townspeople withdrew, promising to brgin the tower down upon them if they did not surrender, but they defended themselves better and better by hurling the huge chessmen down upon them. (453-455)

From Chrétien De Troyes' Arthurian Romances. Translated by Carleton W. Carroll.