You can send the Special Summoned monster to your deck with "Test Tiger" to get another "Gladiator Beast" from your deck, to avoid it being sent back to the deck when this card is no longer on the field.
Chain "Book of Moon" to this card and target this card, then the Special Summoned monster's effect will not be negated.
The special summoned Gladiator Beast can use its effect of summoning another Gladiator Beast if it attacked before Darius and activates its effect first. This is because Gladiator Beasts effects that summons another Gladiator Beast requires them to return to the deck and leave the field as the Cost. Once they leave the field, their effects can no longer be negated.