A gladiator was a type of fighter.
A Shad'ath gladiator was one of the many creatures captured by the Zygon warlords, Anktra and Kestral. They planned to carry out a traditional duel on the planet Shontaa, involving fighting to the death using this variety of host bodies. This duel was observed, however, by the Fourth Doctor and Leela, who found the Zygon ship and freed the prisoners. (COMIC: Rest and Re-Creation)