Gather Specimens

Gather Specimens won't retroactively change the control of creatures that have already entered that turn.

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.

If the effect that puts a creature onto the battlefield also creates a delayed triggered ability, Gather Specimens doesn't change who controls that ability. In the unearth example above, your opponent controls the ability that exiles the creature at the beginning of the end step. On the other hand, if the effect that puts a creature onto the battlefield grants a triggered ability to the creature (with "gains" or "has"), the player who controls the creature at the time the ability triggers will be the player who controls that ability. For example, if your opponent casts Makeshift Mannequin and you cast Gather Specimens in response, the creature will return to the battlefield under your control with a mannequin counter and it will have the ability "When this creature becomes the target of a spell or ability, sacrifice it." If the ability triggers, you'll control it, so you'll have to sacrifice the creature.