Gresham's Law (or, rather, the principle behind it) would hold even in the case of a floating exchange rate (for example, Bitcoin vs USD). I have explained this
here (and in greater detail further on)...
In the case of truly floating exchange rates, when A's price drops against B, you could argue that A is now undervalued, but you could also argue that A was never worth as much as its old price. This is just like any other market, and there's no clear answer. When the state is supporting an exchange rate, and the market leans one way, the state must lean the other way. There is no question which currency is undervalued, and which is overvalued. Gresham's Law concerns itself with state intervention, which is distinct from the free-market scenario.
In fact, you didn't say anything new beside what has already been said in the thread by the link I provided. We have agreed there that we would call this an expansion of Gresham's Law (for the convenience sake)...
Personally, I think that Gresham's Law is primarily about one money driving out another from circulation