I think it is unwise to guess at an attacker's skill level. What distance are you from the attacker? What is the distance to cover? What are your physical abilities? What is the attackers build? What weapons do you have on you? What is your level of training and skill? What is the attacker's demeanor? Is the attacker actively shooting at other people already?
Without assessing all of those variables, I think it is unwise to offer any advice. If an attacker has you at gun point, he already has the edge on you. All it takes for them to shoot you is moving their finger while you have to make it all the way to cover. Any sudden movements may cause him to simply react and shoot you.
If I was in a situations where someone pulled a gun on me, I would comply as much as possible until I had an opportunity to react. It is better to seem scared and let them try to relax as much as possible. It is much better to replace any material possession than to get shot and possible killed or handicapped. If you feel the attacker is going to shoot you no matter what, then fight like hell. If the attacker makes a mistake and gets distracted, then you can consider a counter attack or draw your own weapon. If you draw your own weapon, you better be ready to use it. Do not stop firing until the attacker is incapacitated or drops their weapon.
If you have a decent level of training and feel comfortable in your skills, you could consider counter attacking your attacker if you are close enough. The most important thing is get get out of the line of fire and try to control the weapon. There is no such thing as a fair fight so go for the eyes, groin, throat, knees, whatever is most vulnerable. Once the attack stops, do NOT stop attacking until the attacker is incapacitated or you have secured their weapon.
I honestly feel that carrying a weapon without any hand to hand combat training puts you at a serious disadvantage. Practice often and at full speed if possible. Even then you really need to think before you act!
The ATF should be a convenience store instead of a government agency!