The Gladiator is the informal name given to Myzax, a terryifying and undefeated gladiator in the Galra prisoner fighting ring. Shiro was forced to fight him in his imprisonment with the Galra. It used to be a monster, with only a staff with an orb as a weapon, but Haggar decided to increase his strength with quintessance, in order to make him a threat to Voltron. This turned him into a Robeast.
The Gladiator used to be a monster that fought for the Galra Empire against prisoners of the Galra. He was supposed to be the opponent of Matt Holt when Shiro saved him by injuring him. The Gladiator was undefeated, but Shiro managed to defeat him by recognizing that the weakness of the weapon was after it's third strike.
Shiro managed to defeat the Gladiator and earned the nickname "Champion" as a result. Shiro learns all of this from an escaped Galra prisoner.
Myzax had a large staff, that could generate an orb at will. The orb was directed using the staff, and is visibly indestructible. There is also a good chance that the orb is made of quintessance.
The main weakness of the staff (which reappears later when Myzax is a large robot) is that after the third use, it must recharge. This is the time to strike, because Myzax has little defense past his staff. After Shiro learned this, he took advantage of the flaw in the design to strike the monster, defeating it with his simple weapon.
This upgraded version of the robot is much more reliant on defense, and is built to stop attacks, even from the massive robot Voltron. Because of this, the robot can take multiple full - force punches from Voltron, even sustaining damage from his special attacks in order to retaliate with his orb. Even Voltron's shield cannot stop the orb full - force.