can pack of onders used during a showdown

No — Pack of Wonders cannot be activated during a showdown.

Its ability ([E]: Return another friendly gear, unit, or facedown card to its owner's hand) is a standard activated ability with no Action or Reaction keyword, so it can only be used during your Main Phase in a Neutral Open State (Rules 145.2, 381; FAQ #9558). During a Showdown State, only cards/abilities with Action or Reaction timing can be played or activated (Rule 343.1). You'd need to fire it during your own Main Phase before the showdown begins.