The fumes are bad and I don't like to get it on my hands, but my main concern is flammability. People get badly burned doing stuff like that with gasoline all the time.

Of course, people are also injured in bike accidents all the time, so it would not be unreasonable to say that flushing a freewheel with gas is just one more small added risk, especially if it's something one does infrequently. But given that safer and equally ineffective solvents are available (Pastor Bob is correct to point out that a solvent flush is perhaps better than nothing, but it's not a proper fix), I don't think it makes sense to use gasoline.