Is Gangplank's +3 might effect until the end of turn?

Yes. Gangplank, Naval's Empowered ability grants "+3 [S] this turn instead" — it explicitly says "this turn," making it a "this turn" effect.

Per Rule 317.2.c, all "this turn" effects expire simultaneously during the Expiration Step of the Ending Phase. So the +3 stays with Gangplank for the remainder of the turn it was granted, then wears off when your turn ends (before the opponent's turn begins). The errata from July 2026 added the "this turn" wording to make this exactly clear.