You can find it in the Extras of your gog Rebuild page

You can ear it here too: https://rupertlallyespenjjrgensen.bandcamp.com/album/rebuild-gangs-of-deadsville-o-s-t


Bye the way, thanks to the devs too, I love the game and wish a remake,
the same with 3D animations (if they don't harm the game) and some improvement in the IU
and maybe some balance (that I found already very good), or mod help
a Rebuild 3.3