It worked great in editor, but in packaged build gamemode was not loaded, logs stated:
LogLoad: Warning: Failed to load game mode …
LogStreaming: Error: Couldn’t find
file for package…
This means gamemode is not packaged into build. To prevent this issue one must reference gamemode somewhere. Or easier is to open project settings: Project - Packaging, Additional Asset Directories to Cook and add folder of your missing game mode (for example: “/Game/Player/Blueprints”)
Hope this helps someone in future.