why does the Green Knight give Gawain the girdle?

The Lord's (Green Knight) wife gives Gwain the girdle. He has accepted hospitality in their home, and in return Gawain has made a bargain with him. The lord decides that the next day, Gawain will stay in bed until attending high mass and dinner with the lady of the castle; in the meantime, the lord himself will rise at dawn to go hunting. He suggests one more thing: whatever he wins in the forest tomorrow will be given to Gawain, and in exchange, whatever Gawain wins in the castle during the day he must give to the lord. Gawain agrees to this bargain. This goes on for three days, and each day a similar bargain is made. The last day, Gawain receives the girdle from the Lord's lady and keeps the girdle instead of the bargain. This was a test, and Gawain failed the test and did the dishonorable.