Gravis didn't concentrate on anything specific. He simply looked at the gasses, the stones, the lava, the fire, and basically everything he could find. As long as he saw that he didn't understand something, he started looking at it closely.
Other beasts and maybe even humans might think this was a waste of time, but Gravis didn't think so. His goal was the peak, and to reach the peak, he probably needed to understand everything either way. Understanding it now or later made no difference. Additionally, he couldn't know if something would be necessary for another, more battle-relevant Law before knowing it.
Gravis remained inside the first volcano for three years. Even though he only had 26 years remaining now, he didn't mind his slow progress. Everything would fall into place later.
The second volcano only took Gravis two years to thoroughly check out. He had learned most of the things he could learn in the second volcano already in the first one.
