If somebody’s pointing a gun at you in an enclosed space, you want that gun offline. Away from you. THEN you can use your close quarters battle techniques to fight back. If you end up in a tiny space with a gun pointed at you, don’t make the situation worse by trying to push the muzzle down.
You want to move the muzzle off line to the side, away from your femoral artery and away from your pelvic girdle. In tight and close, grab the gun and move it off your center-line to the side. There are any number of good weapon disarm maneuvers from that position—and if the gun is fired, the bullet will not likely hit you.
Remember Rule #1: Don’t get shot. Pushing the gun down keeps it pointed at you longer than it has to be.