"gold" can symbolize a lot of things, especially in this book. What do you think of when you think about gold? shiny, money, rich, pretty? You can make the argument that "gold" symbolises the idea of purity. "Stay gold" can mean "stay pure" in relation to the gangs thoughts and their friendship, but also staying pure by not letting violence and negativity jade them.
Specifically when Ponyboy is told to "stay gold" by a dying Johnny, it is meant in the first way: stay pure and innocent. Johnny wants him to remain the good, moralistic character that he is. Johnny is saying this, not strictly as a slang term, but rather going back to the Robert Frost poem Ponyboy told him.
In the book "The Outsiders" when Pony Boy and Johnny are hiding from the law in the old church Pony Boy reads a poem to Johnny. Johnny later contemplates on the poem and before he dies he leaves Pony Boy a note. Pony Boy finds the note in a book and the nurse had helped Johnny to write it. In the note he tells Pony Boy to stay golden.
Johnny says in the book that he had figured out what stay golden had meant. He believed that it had meant to stay innocent like the little children. He wants Pony Boy to stay innocent and not become hardened like the rest of the Greasers so that he will use his talents to be different than the other Greasers. Johnny has realized that all the fighting and drama and not worth anything.