the act of disposing of it (for fiat or anything else) is what creates the capital gain. it doesn't matter if you sell it for 'fiat' currency, goods, services, land, or anything else.

the mistake people keep making is to assume that a nation's fiat currency has any special place in computing the timing or amount of capital gains, except as the denomination of value in which the tax is paid. the way the tax laws work is much simpler: if you buy something for something worth $X and trade it away for something worth $Y, you owe $(Y-X) in (capital gains) taxes.  (modulo, of course, the exceptions you mentioned, like bands of tax-free income for small amounts.)