Gladiator Superman

Superman's barely-there gladiator costume is hardly more than a footnote these days, although it did appear in a one-panel flashback in the aftermath of Superman Reborn (Action Comics #978, 2017) so we know it's still canon.  This variation of Superman's costume is both ahead of its time and dated, from a strange period in the 1980's when homoerotic action figures like He-Man were all the rage.

Action Comics Annual #2

Action Comics hasn't been part of this story line since the book went weekly, but the 2nd Action Comics annual makes up for its absence in a 48-page story kicking off a new direction in Superman's Exile epic.  There are lots of artists handling the duties here:  George Perez designs the gladiator costume, and Mike Mignola is once again the definitive post-Crisis Krypton flashback artist.

Mongul holds Superman prisoner on Warworld and forces the last Kryptonian to fight to the death in gladiator games.  Superman keeps winning the fights, but refuses to kill his opponents, which infuriates Mongul.  Discovering that the new contestant is Kryptonian, the cellkeeper secretly contacts an alien Cleric who it turns out had been on Krypton centuries ago.  Reaching through space, the Cleric merges his mind with Superman's, confirming his implanted memories of Krypton and also sharing his own memories with the Man of Steel.  Centuries ago, the Cleric had tried to take an ark of refugees off of Krypton, but they all died due to a genetic defect (which, is also explained, Jor El had to cure to order to successfully send his son to Earth).  The Cleric also reveals that he still possesses a powerful Kryptonian weapon called the Eradicator, which will play a major role in future Superman stories, including his death and resurrection. 

Superman #32

After Superman refuses to kill Warworld's defeated champion, Draaga, Mongul teleports into the arena to kill them both.  Having been away from a yellow sun for too long, a weakened Superman speculates he might have difficulty beating Mongul even at full power.  Despite defeating him, Mongul deduces that if the Man of Steel could survive his bare hands, it might be embarrassing if he fails to kill him publicly with his blade, so he removes both gladiators from the arena to dispose of them in private.

On Earth, Matrix wanders lost around Metropolis in the form of Clark Kent, until Jimmy Olsen spots him and takes him back to the Daily Planet.

Later in the torture chamber, Superman breaks out of Mongul's restraints but once again is no match for the tyrant's brute strength.    Mongul blasts Superman with the amulet on his chest, seemingly incinerating the last Kryptonian.  But Superman has really been saved by the Eradicator and transported into the Cleric's arms, cradled in a pietà.

The Adventures of Superman #455

A revolt has broken out on Warworld now that somebody has dared defy Mongul, even though the challenger is now presumed dead.  Nevertheless, the overseers of Warworld promise Mongul's throne to Draaga if he will dethrone the tyrant, so he confronts his master for a rematch.  Draaga rips Mongul's amulet out of his chest, and the wounded Mongul flees from Warworld.  Draaga wonders if they will all meet again someday.

On Earth, Matrix acts strange as Clark Kent and doesn't seem to recognize any of his friends.  Cat Grant visits Lois to reveal that she's been Clark's inside source in the Intergang story, which is the real reason why she's been getting close to Morgan Edge.

The Cleric revives Superman and shows him the Eradicator.  Bonding with the artifact, it first transforms Superman's clothes into traditional Kryptonian garments.  Once he figures out how it works, it restores his Superman costume, and that's the last we see of the gladiator outfit.