Gorgosaurus is a genus of theropod dinosaur that lived in western North America during the Upper Cretaceous , between about 76.5 and 75 million years ago . Gorgosaurus ( , meaning `` fierce lizard '' ) is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period , between about 76.5 and 75 million years ago .