The “Gladiator Beast” cards returned to the Deck are returned when the effect of “Gladitor Beast’s Respite” is resolved. It is not a cost.
You can return any card with “Gladiator Beast” in its card name to your Deck when resolving the effect of “Gladiator Beast’s Respite”, such as “Gladiator Beast’s Battle Halberd” or “Gladiator Beast’s Battle Manica”.
When resolving the effect of “Gladiator Beast’s Respite”, if you do not have 2 “Gladiator Beast” cards to return to your Deck the effect of “Gladiator Beast’s Respite” disappears. In this case you would not return any “Gladiator Beast” cards to your Deck and you would not draw 3 cards.