The one true way to stop a gap heat is to have enough men blocking so your able to defend the gaps in the offensive line.
Lets say you running Gun normal HB on the strong side. A gap heat isn't a problem when your in the Gun you have at least 3 to 5 sec to get the ball off. The problem is the way you read the heat and how you defend it. What do you change; which way do you line block? left, right, down, up? Now if you block left your line is strong to the rush from the left but weak to the rush to right and vice a versa.
Let say you read the right DE is out wide on your left tackle, now you know by this set up that your left guard can't block two guys so A gap heat will kill your QB so how do set your line to block? What I do is either pinch my line or block to the right. The main reason I do this is 2 fold; 1. You can set the pocket up to your strong side 2. Your running back is line up on the strong side in this set, and if his route is to the flats its a better read for your QB plus you defeated the blitz.
Remember its much more about moving the chains than taking a loss. That's why I almost don't ever fan my line out, this will shut your drive down with a sack from A gap heat. You must remember the routes that your players are running just incase you have to rush your throw. In some cases you could fan your line out but I only do this when I have HB or WR running crossing routes over the middle and I need to clear out traffic for a easy gain.
Remember throws to the flats for a easy gain or wait for a WR to beat his man, but that doesn't always happen. You have to force the defense to rush more men to try to get to your QB thus making it easier for the offense to move the ball and win.