I assume that no one was hurt. That is typical. The BGs want the money, minimal hassle, and minimal danger to themselves. The cowardly and selfish nature of criminals means that they generally don't want to use their guns for anything more than intimidation.
Unless something causes me to think that they intend to use their firearms, evasion is best. However, in the most likely scenario, evasion will not be possible. Like all the other customers, I will likely be caught up in the situation. If they have not already noticed my firearm, I will comply with their instructions while keeping my strong side from their view. Until something happens that makes me think that people need defending, I will merely take in as much information as I can so that, should I need to act, I will be fully aware of all threats and where the innocents are.
If the BGs have noticed my pistol, I suspect that one of two things will happen. Most likely, if things have not gone too far, they will leave. Less likely, they will have the proverbial "drop on me," in which case compliance creates the greatest chance that no one will be shot.
It's just money, and most bank robbers are caught rather quickly. Unless there is threat to life or limb, I won't try to use my gun.