As is often the case, there's more than just one aspect of this to consider.
From a legal standpoint, a robber pointing a gun at you is almost certainly going to provide justification for the use of deadly force. In other words, if you do end up having to use deadly force, the fact that a robber pointed a gun at you would be enough to satisfy a judge or jury that you were legitimately in fear of being killed or seriously injured.
From a survival standpoint, reacting immediately with deadly force to a gun pointed at you won't always be the best strategy. As has been pointed out, outside of military actions or premeditated murder, guns are more often used to intimidate/coerce/deter than to kill or injure. Taking the blanket approach, by responding with deadly force to any pointed gun, could actually reduce your chances of surviving/remaining uninjured. You'll have to assess the circumstances of the situation to determine what you feel makes the most sense, but you don't want to stop thinking and begin an automatic/reflex type response just because you see the muzzle of a gun. It's easy for us to assume that once we make the decision to draw and fire that it's all going to be clear sailing, but the outcome for defenders isn't always positive--they aren't bullet proof superheroes. If it looks very likely that an armed robbery can be resolved without shooting, then not shooting is probably the safest strategy from a survival standpoint. When the shooting starts, there are no guarantees that all the bullets will end up in the bad guy. Defenders (or bystanders) can also end up seriously injured or killed.
John nailed it. There have been 2 separate issues conflated together.
When a gun is pointed at you, legally, they have demonstrated intent to put you in fear for your life and you have the right to act in self-defense.
However, from a survival standpoint, if they will be happy with your wallet and leave, give them the wallet. If they just popped a dude next to you...fight! If it is in between those 2, make your best decision as it goes and be ready to act in an instant.
Drawing on an already drawn gun is a bad deal, but how many people train gun disarms and/or to strike someone holding a gun successfully (while clearing the field of fire) or strike and draw/shoot from retention?