The first Law Gravis comprehended was also the very key to all the other Laws that would follow.
It was the level four Law of Microorganisms, also called the Major Law of Life Energy.
Back in the middle world, Gravis had comprehended the level one Law of Microorganisms after comprehending all the level one Composition Laws of the Body and Plants.
This level one Law had been the key that bound all the different concepts together, allowing Gravis to comprehend the level two Laws of Body and Plant Composition back in the middle world.
Life Energy was very different when it came to Immortal Cultivation, and the biggest difference was the inclusion of a soul. The soul in the microorganisms allowed the inherent soul-energy inside the body to also regrow.
After Gravis comprehended the Major Law of Life Energy, all the concepts regarding the Composition of an Immortal body clicked into place.