Now lets get back to what the OP in fact said. The thug approaches and makes his threat i.e. lifts his shirt revealing a gun. This is an armed robbery, this is not this creeps first offense he is a predator of the vulture jackal variety not a lion, a bully not a warrior. He lives on intimidation, threats. He is there to take what he wants from me, not do battle. That is the LAST thing he wants or needs, He has picked me as prey, someone he has decided is weaker than he. A sick, cowardly, disabled or otherwise vulnerable victim. A target He EXPECTS compliance from, otherwise he would have never ever have approached me.
He assumes I am intimidated fearful, he does not expect any other response, his gun is in his pants Mexican carried. He has not drawn his gun, YET. He has let me know that he is willing to kill or harm me, has the apparent ability to do so and has taken substantial steps towards doing that.
Now is the time to act.
Per the scenario presented,
Spit on him?
Draw a knife on him?
I don't bring spit to a gun fight
I don't bring luggage to a gun fight
I don't bring fists to a gun fight
I don't bring knives to a gun fight
An aside I carry right above my rear pocket right above my wallet, when most anyone approaches me I greet them bladed. In part because, a robber seeing my hand go to my rear pocket will most likely assume I am getting my wallet. Not a .45 I intend to do the unexpected. Punch .45 holes in is chest.