It is arguable that "gradually" is used correctly. But it is kind of subtle. Probably most readers would not be bothered by it.
"Gradually" indicates something changes slowly. You can gradually slow down a car. You can gradually increase the temperature in your oven. You can gradually increase the depth of water in a bathtub. Note that all of these examples are continuous.
However, "shooting" comes in discrete shots. Each shot is a discrete event. So it is a little bit odd to say that something that comes in individual discrete events could gradually stop.
Also, "stopping" is not a gradual thing, but an end point. You could say "he gradually moved along the couch, eventually arriving at the TV remote control." But you would not say "he gradually arrived at the TV remote control." Here too, you want "gradually" to apply to something that is continuous, that can change slowly.
So you might say something like "the rate of shooting gradually decreased then stopped." Or you could say that "the shots gradually came farther apart then stopped." Or you could say "the shooting gradually tapered off then stopped." That is, the thing that is changing gradually would be something that is continuous.
But, as I said, most readers probably would not be bothered by it.