Gawain learns a lot at the end of the tale Gawain and the Green Knight. He learned that the knight knew when he hid the girdle from him. They had a deal to exchange what they got at the end of the day and Gawain hid it from him. He also learned that the house he was staying in actually belonged to the Green Knight himself. The knight told him that the third strike that cut his neck was because he didn’t tell him his wife had given him the girdle for his protection.
He also learned that his host was the servant of the old woman the knight had told him to spend time with. Gawain learned that he must be an honest knight. He held the girdle because he was selfish and wanted protection. In the end, his dishonesty taught him that he shouldn’t have been so selfish.
I think what makes Gawain heroic is that he was willing to come forward and admit that he was at fault. He made a vow to be an honest man from that day forward. I think it was very courageous for him to admit fault and own up to his dishonesty.
robbselit 
You make a really good point that part of Gawain’s problem was that he was selfish in accepting the girdle. He resisted the Queen because that was actually easy, but resisting the temptation to cheat in his deal with the Green Knight was harder for him. I think you are right that his admission of fault was his act of bravery.