Gresham's law requires two currencies to be a legal tender at the same time. One good, one bad. Legal tender means that all businesses must accept it as money. In that case people think "this first thing is actually better than another thing, but the state forces the grocery store to accept both at the same rate, so I will spend the bad thing and collect the good thing".

So, the business has to be forced to accept both. I think, this is what happens in El Salvador. We can check the statistics on how frequently people pay with dollars and BTC. The business is forced to accept both, as is required for Gresham's law to apply.