Some effects that put creatures onto the battlefield continue to affect those creatures later on. Although Gather Specimens changes whose control the creature enters under, the rest of the effect works as normal. For example, if your opponent activates a creature card's unearth ability and you cast Gather Specimens, that creature enters under your control, but the rest of the unearth ability is unchanged. The creature has haste. It's exiled at the beginning of the end step. If it would leave the battlefield, it's exiled instead of being put anywhere else.