You exhibited a small amount of concern, in my opinion. Enough to go on alert and place your hand on your weapon. To me this is not overreacting. My questions is...would you have reacted exactly the same way had the subject been a 22 year old, clean cut white guy?
It's defensible to say one is more concerned by a young black man doing something to raise red flags. Statistically young black men are more likely to be engaged in criminal activity. But as noted simply having a gun is not proof of any wrongdoing.
I have spotted guns being carried numerous times. I don't recall ever feeling particularly threatened, but I'm not a cop and I am also very comfortable around people with guns. Almost casual in my attitude.
One incident I recall was about 20 years ago. Two men in their 30s were loading groceries into their car. I believe they were a couple. They sort of gave me that feeling and the gay pride bumper sticker was a tip off. As one of them leaned into the car his sport coat rode up and partially revealed a holstered semi-auto. The Second Amendment isn't just for rednecks.