The lion, a creature that should have certainly been badly wounded by the blows that Gus had dealt to it, inexplicably leapt at Gus. He found himself unable to move, perhaps out of the disbelief of the situation that he found himself in. As the creature came towards him he noticed that the creature's side, the area where he had shot the weapon at it, showed no signs of the shot even being fired. There was no blood that had escaped the creature's body from a wound, nor was there a wound in the first place.
Gus was unable to move as the creature leapt, thanks both to the surprise of the creature's movements as well as the fact that his feet were not positioned well enough to allow for him to avoid the beast's attack. All he could do was brace for the attack to come in and hope that it wasn't something that would leave him too badly wounded.