If someone is holding a gun on me, I can safely make one of two conclusions:
1) They're bluffing, and don't actually intend to use it for anything other than a threat
2) They intend to use it and kill me with it once it's all done
Either way, it's not in your interest to do nothing. The end result with freezing is that a person ends up either dead, injured, crippled (can't forget the 'less than binary' potentials!), or emotionally/psychologically victimized. There is no positive ending for you if you just give in.
So, provided a person doesn't freeze up instantly when a gun is pointed at them, chances are they're not going to shot. In other words, acting as if you don't even notice the gun, or freaking out and moving either agressively or defensively, will be to your greatest advantage. If you're within 10 feet of the guy, your response can likely be fast enough before he fully realizes what's happening, provided you're on your feet; chances are he's got a pre-conceived notion of how the robbery is going to go down, nad if you don't behave according to that notion, it'll take a while to adjust to it.
Even if you momentarily freeze, you could say, "holy cow John, you scared the bejeezus out of me! that's quite the joke!" meanwhile you're holding your gut as if you're recovering from being startled, maybe you're walking towards your car and 'fishing for your keys' (and when your gun hand is concealed slowly turn and pop one or two in his side) or bending backwards as if you pulled your back (to grab your CCW). Something, anything - just don't play it straight.
The basic point I'm trying to make is that the agressor is expecting a victim. Complying, or intending to comply and pulling a fast one on him is exactly what would likely be expected for a victim to do. Don't act like a victim, act (and think!) like an agressor yourself!
This is part of the reason why I keep my CCW in a location where, if I were to need to take out my wallet, the pistol could easily be drawn with the other hand without it being noticed. Slight of hand...