Version 42416, with the added Co-op and New Game+ features, is now out on GOG for Windows, macOS, and Linux! We're sorry this build was delayed on GOG, and hope to be back to roughly concurrent releases for Steam and GOG in the future. You can see all the patch notes for this update here:

https://store.steampowered.com/newshub/app/421170/view/2221906285538975514

Robert Thomas
Lead Programmer
Lab Zero Games