The knights of the round table express their solidarity by wearing a green girdle as gawain does. why does gawain wear the green girdle?

The green girdle was given to Gawain by Lady Bertilak. She gave it to him as a lover's token. She also mentioned that the wearer will become invincible.

For Gawain, it is a symbol of his survival at the expense of his code of honor. When he failed the Green knight test, he wore his green girdle to symbolize his failure and be a reminder for him of a lesson learned. 

When the Knights of the Round Table wore green girdle as a sign of solidarity, it symbolizes the men's honor.  

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Lorna would be the complete subject in this sentence.