I believe "when it's gone, it's gone" it's an expression, the "it's" refers to an object not to a person. "when" is something that will happen in the future. The meaning would be, that when the time comes to use that specific object then you will use it. A more used expression, simillar to this one would be "what's gone, it's gone", meaning that you lost something in the past that you cannot bring back.

It's more of a statement used when something is given to another, even more specifically if there are very few of this item in existence. For example, your grandfather gives you $100, and tells you to spend it well. The correct expression would be "When it's gone, it's gone.". The statement after the comma is just confirming that it will indeed be gone, when you lose it/spend it/ etc. It's not a very common expression.