The piano is in the Copenhagen map, in the attic area where you place planks to progress. One Reddit report says you can hear it faintly near a barricaded door next to a deceased body, and the sound gets louder before stopping.

What players usually mean

People asking about the piano in Guts & Blackpowder are usually referring to that hidden ambient piano in Copenhagen rather than a usable instrument. It seems to be part of the map’s atmosphere and lore rather than an interactable object.

In-game context

The game also has music-related content tied to the Musician class and instruments like the fife, drum, bugle, and bagpipes, but those are separate from the Copenhagen piano sound.

TL;DR: Check Copenhagen’s attic/barricaded-door area; the piano is an ambient sound cue, not a standard pickup or weapon.