There are other gem kings with the power of the C grade, but the two of them are currently the strongest gem kings, which makes them the the strongest manpower that the gem civilization is capable of mobilizing.
The Gem people are not a race that is inherently violent. They like sleeping and can be peaceful. But once the fight for precious material comes up, the Gem people become capable of violence.
After they get what they want, they usually return to their peaceful nature. This is clear by the fact that they haven't tried to kill anyone else after they killed the Snow King.
The gem people were content with gaining the support of the Sun King for mining Aries and transporting the precious materials back to their world. They were ready to bury the hatchet of war and returning to their peaceful lifestyle.
