The first Law that Gravis decided to comprehend was the Law of Frost's Cold. Just like the Law of Inferno's Heat, the Law of Frost's Cold was an element-specific temperature Law.
This was exactly the Law that Samantha had used back when Gravis fought her. She had been able to unleash the Law as a domain, but with her Weapon Techniques, she had also been able to concentrate the power of this Law into her rapier.
Gravis had decided to understand this Law in case he fought someone with the Inferno Element. The Law of Inferno's Heat would obviously not be as effective against such an opponent. Additionally, the Law of Frost's Cold had a different secondary effect, which was very important to Gravis.
The Law of Inferno's Heat would damage an opponent and force them to use more Energy to protect themselves. One could say that the Law of Inferno's Heat had greater destructive power than the Law of Frost's Cold.