This happened to one of my friends when he was doing an internship in Dallas... he was out looking for a cab after the bars one night, and someone walked up, put him at gun point, and demanded his phone and wallet.
My friend said he followed his first and only instincts in the situation: he punched the guy in the face as soon as he was done asking for the wallet and phone, the guy dropped the gun and hit the deck, my friend grabbed the gun and put the other dude at gun point until he ran away.