In Sir Gawain and the Green Knight, there are two main games: the initial Beheading Game, where the loser is the one who dies from the return blow in a year, and the castle's daily Exchange Game, where Sir Bertilak hunts and Gawain receives kisses from the lady, and they swap their "winnings" (game). Winning means upholding your chivalric vow (like honesty), while losing involves failing a moral test, as Gawain does by hiding the magical girdle from Bertilak, leading to a nick on the neck instead of death at the final blow.
1. The Beheading Game (The Green Knight's Challenge)
The Setup: The Green Knight challenges Arthur's court: any knight can strike him with his axe, but in a year and a day, that knight must receive an identical blow back from the Green Knight.
How to Win/Lose: Gawain strikes off the Knight's head, expecting him to die, but the Knight lives, proving Gawain met the terms. The game is won by Gawain initially by fulfilling the strike, but the real test comes later when he must face the return blow, where honesty and virtue are tested.
2. The Exchange Game (The Castle's Game)
The Setup: At Lord Bertilak's castle, a second game is proposed: each day, Bertilak gives Gawain whatever he hunts, and Gawain gives Bertilak whatever he wins in the hall (kisses from the lady).
How to Win/Lose:
Winning: Gawain wins by receiving kisses from the lady and giving them to Bertilak, fulfilling the terms.
Losing/Failing: Gawain loses by accepting a magical green girdle from the lady on the third day, promising to keep it secret to save his life from the Green Knight's final blow, thus breaking his honest exchange with Bertilak.
The Ultimate Outcome
Gawain loses the "game" by keeping the girdle secret, revealing a flaw in his perfect chivalry. The Green Knight spares him, giving a slight nick as a reminder, revealing he was testing Gawain's honesty and courage, not just his skill.
Sir Gawain and the Green Knight - Wikipedia
(also) Shmoop (as a reference)