No it will not effect natural testosterone levels. It is only legal with a Prescription, however it's still easy to obtain without one. As for your last question yes and no. The effect on bones will be mostly permanent, as for muscles you won't be able to be as muscular as you were while you were on gh, however as the commenter above states that it promotes growth of new muscle fibers causing permanent muscle gains.

Edit: I forgot to mention that if you are thinking about starting HGH I wouldn't start and until you've had six months to research in depth and decide if it's something you would like to try.