The special is not actually Episode 377, but a standalone 15th-anniversary TV special titled One Piece "3D2Y": Overcome Ace's Death! Luffy's Vow to his Friends . While the original Episode 377 covers the aftermath of the Thriller Bark arc, the 3D2Y special focuses on Luffy's two-year training period under Rayleigh on Rusukaina. Where to Watch Legally

There are no official "free" streaming options available for the at this time. You can watch it on these major platforms:

Watch One Piece 3D2Y Episode 377 for free on go... Ghostring Card 1 obtained.