"Gladiator Beast" Fusion Monsters cannot be Fusion Summoned. Their texts say they "can only be Special Summoned from your Fusion Deck by returning the above etc.", and while normally "can only be Special Summoned by" means "the first time you Special Summon this card it must be done this way...", there's no way to Fusion Summon them. That's because whenever they return to the Fusion Deck their effect is "reset", so that you have to perform the method for the initial Summon again (i.e. the returning the cards to the Deck thing).
Moreover, you can only use a card like Monster Reborn on a Fusion Monster that has been properly Fusion Summoned and then sent to the Graveyard. "Gladiator Beast" Fusion Monsters are not initially Fusion Summoned, so if they're then sent to the Graveyard you cannot Monster Reborn them.
Essentially, the only time you're ever going to get a "Gladiator Beast" Fusion Monster out is if you do what the card says to do (the Contact Fusion process, as the anime calls it).