I want to diversify my savings portfolio and gold is an option. I dont live in the US and im not a citizen, how are capital gains managed if I only sell 1 time in my life

we don't give investment advice, and it would seem you would not be subject to US taxation unless something changes. 

"how are capital gains managed"

What do you mean by "managed"? Are you asking about reporting capital gains on a U.S. income tax return? If you don't live in the United States and you are not a U.S. citizen, why would you be filing a U.S. tax return? Is there something that you haven't told us about your situation that would make your personal sale of gold subject to U.S. income tax?

This forum is for questions about using TurboTax software to file a U.S. income tax return. If you are not asking about a U.S. income tax return you will have to look elsewhere for an answer.