The ideal Christian knight would have to live up to the code of chivalry. He would have self-discipline when it came to lust or physical pleasure. He would hold in high-value physical courage, loyalty to the lord or king, and piety toward God and Christianity.
Sir Gawain didn't fail to live up to his ideals. He made a small mistake, he should have told the host about the wife’s attempts to seduce him, instead of hiding the truth from him, but the Green Knight sends him on his way with his knightly honour intact.
Sir Gawain learnt that no knight is near being perfect without making any mistakes.