Loot a nearby store while everyone is distracted?
Seriously though; since my training has taught me how to disarm an opponent (British Army and security course) - I'd stay still, hands up and talk to the fellow.
Yes, disarms do work occasionally. But in a mall where someone has the drop on me from relatively close but not point-blank then I don't favour my odds. Though you can think otherwise, I say as one trained individual, it's not a good idea.
Trying to talk the man down, however, is a very good idea. I know that if I reach for any sort of weapon (not allowed them in the UK alas) or blunt object I'm getting a chest full of lead before my hands get anywhere near my stuff.
The mall will be in panic, so the bloke might be distracted - but the OP points out the gun is in your vicinity, and you're in the line of fire (so it's reasonable that his aim is going to come back to you, as you're a central point).
I'd strike up a conversation, moving slightly to shield loved ones (and try to get them to back out the way - given most of the mall is running around screaming it should be OK) but turn slightly sideways and be ready to drop into a roll towards the nearest cover (minimising my profile as much as possible at all times, so I'm harder to hit - even if only slightly so). Either he backs down or I try to get out the way - and sit behind a nice marble potted plant and wait for one of the other many, many heroes present in society (via this poll) to beat him up.
Then change my pants.
So a little of all of them really, so clicking "other".