In Sir Gawain and the Green Knight, what does Gawain have as a reminder to always do the right thing and never fear?

The green girdle.

Explanation:

Sir Gawain's faith is tested by Lady Bertilak, who offers him her green girdle. This bonds both elements: chivalry and love. Over the poem, this girdle has an evolution of meaning forGawain: even if it's just an object, a piece of cloth, he deposits all his faith in it, making it not just an object but a representation of virtue and humility, a reminder to do the righteous thing, to not succumb to fear. In the end, this symbol of virtue is worn by the entire court.