Does a 20 amp GFCI outlet on a 15 amp circuit pose an inherent danger?
First off, connecting a 20 Amp load to a 15 Amp circuit will overload the circuit but shouldn't cause a fire because the 15 Amp breaker will trip.
A 20 amp GFCI doesn't pose any threat just by being there. The threat comes from people thinking it's on a 20 amp circuit and loading it as such.... which will trip a 15 Amp breaker.