The Metal Ultimate was surprised when he noticed that Gravis paid attention to him first. Didn't Gravis have a lightning affinity? According to reason, he should first take a look at the Lightning Ultimate's display.
The Metal Ultimate didn't know much about Gravis since they were of different camps. Additionally, he hadn't seen Gravis fight anything. The Metal Ultimate had seen the competition between the land Kings, but Gravis hadn't fought in that. Instead, everyone had already accepted that he was the most powerful King.
Then, the Metal Ultimate saw Gravis going through his different kinds of ores and experimenting with them. That was when the Metal Ultimate finally saw the reason for Gravis' power. Gravis had already comprehended a level three Law, the Law of Punishment Lightning!