After leaving Skye, Gravis stayed around for a little while longer, though hidden inside the earth. He had to make sure that Heaven didn't pull anything while Skye was unconscious. When Gravis noticed that Skye regained its awareness, he slowly snuck away. He didn't want to hear its cries or see its pain.
When Gravis sneaked away to a sufficient distance, he sped away with his full strength. His ninth level Destruction Lightning, and his physical body made him even faster than Skye. This was an additional reason why he decided to let Skye go. If he fought an enemy that he had problems with, Skye wouldn't even be able to escape since such an enemy would most likely have an even faster speed than Gravis.
Mass Release over.