The Gladiator is the informal name given to the giant robot gladiator that Shiro was forced to fight in his imprisonment with the Galra. It used to be a monster, with only a stick and an energy ball as a weapon, but Haggar decided to increase his strength with quintessance, in order to make him a threat to Voltron.
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.