How does Gangplank Naval work?

Gangplank, Naval has two pieces: an Empower activated ability, and a replacement effect he gains once Empowered.

• Empower: Pay [C][C] (two Body runes) to Empower him. Can only be used once — you can't activate it while he's already Empowered (Rule 827.1.c.1). The Empowered status itself lasts until he leaves the board or is Disempowered.
• Empowered ability: While Empowered, if a spell or ability chooses him and would stun him, give him −Might, or return him to hand, that effect doesn't happen — instead he gets +3 Might this turn.

So a Stun or bounce attempt on an Empowered Gangplank turns into a pump. The +3 is a "this turn" effect and wears off during the Expiration Step of your Ending Phase (Rule 317.2.c; FAQ #11824). One nuance: the triggering effect must actually choose him — an effect like "stun all units" that doesn't choose doesn't trigger the replacement.