A gadget weapon is a melee weapon that has been modified to allow some new trick or stunt. For example, a Zweihander modified to Impale.
The GM must accept each gadget weapon based on suspension of disbeleif and somewhat on power balance. If this stretches reasonable beleif, the GM may forbit it or say that it jams on any snakeyes roll.
Like all modifications this costs an Items Schtick.