Game does not start after recent update.
Following message pops up:
Fatal Error
Steam must be running to play this game (SteamAPI_Init() failed).
Edit:
Found a workaround on the GOG forums:
Open the file 'ProjectZomboid64.site.json' using notepad and edit this line, from '"-Dzomboid.steam=1", to '"-Dzomboid.steam=0", saved it and fired up the game.