Are you talking about having a national property tax? Won't the states revolt because the feds are encroaching on one of their sources of income?
I don't think gold or jewels are much risk, but there is certainly some. Both increase value as well. Today, gold is down ... according to https://www.jmbullion.com/charts/go...Q7N7ZREHFa2aYDg_2wIJsCoNsyRFCVTRoCgbMQAvD_BwE
Overall, during the past 10 years, it has been up.
Like all assets purchased for investments, the governments will get their money when the asset is sold.
I guess the rich are already paying an additional 5 percent on assets sold.
Gold fluctuates. Take a look at: https://bullionexchanges.com/charts/gold-price. It's a safe place as you said, and if you need to protect your assets, you can do so through physical metals.