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.