Gravis felt great when he realized that he had comprehended the level five Law of Humility directly. Comprehending the apathetic part of the Law of Humility wasn't hard at all since acting like this was very similar to how Gravis had acted when he was younger.
Back then, Gravis wasn't all about comprehending Laws but was all about killing his enemies. He didn't really demonstrate his power to make his enemies go all out but took full advantage of the fact that they were underestimating him.
So, the apathetic part came naturally to him.
However, the empathetic part would be something completely different.
Gravis would never act like he had acted in the Heaven's Trial.
Gravis always took the initiative and dealt with problems directly.
Why?
Because he had the power to do so and because it was necessary to jump into danger to become powerful.