The above procedure means that if two opposing players have each cast Gather Specimens during the same turn and a creature would enter under the control of one of them, it really will enter under that player's control. (The creature would enter under player A's control, so player B's Gather Specimens affects it. Now that creature would enter under player B's control, so player A's Gather Specimens affects it. Each replacement effect has now been used, so the creature will enter under player A's control.)