When a gun is used in a mugging, very important thing to
remember; Your life, more important than your money and belongings.
Give whatever you have and avoid looking at mugger's face.
If he wants to take your life and you know it, take his 1st before
he could take yours - in whatever way you can. But if you're in a
position that you can't assess your chances, pretend to be
frightened and say you have more to give somewhere else, give
yourself chance of escape or change your position.
It's very subjective.
Usually in a scenario this critical, it's the "life and death" mode
we'll be in. So, you need to focus on disabling the threat or
getting away from it any way you can.