In the Green Knight's opinion, Gawain is a good knight. Otherwise, his third strike would have hit its mark and Gawain would be no more.
However, because Gawain confessed his weaknesses and his lie (the green sash was kept secret because he truly did not want to die), he is given another chance. It is understood that he is to go back to Arthur and the round table to tell his story and teach the lesson the Green Knight has taught.
I would also say Gawain is a good knight. He is human, so he has faults. However, he does not act on the temptation of the Green Knight's wife any further than a kiss. Unlike the knight in the Wife of Bath's tale, he does not force himself upon her or harm her in any way. His weakness is his love of life, and going back on the agreement to exchange everything he receives with his host upon his return from hunting.