The “becomes unattached” ability triggers if (a) Grafted Wargear leaves the battlefield, (b) the equip ability moves it onto another creature, (c) another effect moves it onto another creature, (d) an effect causes it to become unattached, or (e) the creature leaves the battlefield.

Note that if (e) happens, the creature won’t be on the battlefield to be sacrificed. In all cases, it’s the creature that was equipped when the ability triggered that will be sacrificed.

If your Grafted Wargear somehow ends up on an opponent’s creature and it then becomes unattached from that creature, you won’t be able to sacrifice that creature because you don’t control it.

You may activate this card’s equip ability targeting the creature that it is currently attached to, even if it’s the only creature you control. Since the equip ability costs zero, you can do this as many times as you like during your main phases, though it generally won’t do anything.