I think that the phrase "stay gold" means that Ponyboy should keep on being the way that he is. He doesn't act the way the other greasers act. He is gold. He does not buy into the "let's fight and rumble" ideas that the others do. I think that Pony sees the world differently than the rest. Johnny's letter helped him realize that Johnny really cared everybody. I think it also helped him realize the importance of Pony's family that he does have. (taken from Yahoo answers, i did not read this book or watch the movie. I do not own this. https://answers.yahoo.com/question/index?qid=20061218184214AASha7E for more detail
