It should be gallant depends on who there asking
Arthur should not forgive Lancelot due to his disloyalty because it greatly affected him and his life. His wife, Guinevere, was having an affair with Lancelot and it took a toll on him. Arthur should only forgive Lancelot if Lancelot earned his trust back once again.
Sir Gawain is loyal to King Arthur because he proved that he can be loyal and chivalrous when no one stood up against the Green Knight. He stood up against the Green Knight despite the fact that he was still not a knight.
Yes, loyalty can be restored even if it was once lost. It can be restored as long as the person was willing to trust the person who betrayed him once again and if the person who betrayed him earned back his trust.
