The "Gladiator Beast" effect returns the cards to the deck (tagging-out)... "if this card attacked or was attacked, you can return it to the Deck to Special Summon 1 "Gladiator Beast" monster from your Deck".
At the point the deck is to be randomized, I see no problem in placing the card in the middle, top, or bottom of the deck, so long as the shuffling is done well. Additionally, the player must allow his/her opponent the opportunity to cut (or shuffle then cut).
However, to the question itself, placing the card into (instead on top of) the deck before shuffling is not a method of stacking or pre-sorting.
Ken Maness
Pflugerville, Tx
RK 2 YGO, PM2, Specialist 2, TO 1