Does Ponyboy learn what it means to "stay gold" -at least according to Johnny

Yes, in my opinion he does. Johnny wanted Ponyboy to fulfill his destiny, to become all he could be. Ponyboy isn't able to pick up the emotional pieces immediately, as he has to take the time to grieve, but he does move on. This becomes evident in his willingness to defend himself, and ultimate choice not to use the bottle he broke with that in mind. He even cleans up the broken glass.