- Sales of gold are already subjected to taxation in nearly every country. I don't know if the government will succeed in taxing BTC. I guess only if BTC needs to get converted to currency, because then the banks and the IRS get involved. But if we can avoid that conversion for payment (directly in BTC), the IRS and the government are bypassed.
AFAIK even though El Salvador and Central African Republic accepting Bitcoin as a legal tender, when you want to pay something directly using BTC, you're still paying in USD equivalent. So until now there's no country set a price in BTC equivalent.
But I don't think paying in BTC equivalent will make you avoid paying tax, because if the banks are accepting Bitcoin, they're will monitor your address and we don't know what they will do. Maybe they will skip if your total holdings are small, but if it's really huge you need to prepare anything.