"When a government compulsorily overvalues one type of money and undervalues another, the undervalued money will leave the country or disappear from circulation into hoards, while the overvalued money will flood into circulation."
I think Gresham's law applies to monies that have predominantly the same properties - like silver coins and gold coins and so compete on an equal footing. However Bitcoin and government money have different properties (for example in how they can be transferred), so they compete on more criteria than expectation of future value alone. The degree to which Gresham's Law holds depends on the similarity of the properties of the two currencies.
tc,du; I might want to keep my Bitcoin, but it might be the only option for payment if I wish to buy a hand-made porcelain bowl from Sudan over the internet.