First run needs on gaming mode requires some work arround:

1. Run steamdeck in desktop mode and launch the game
2. Start a new game or load a save file
3. once you can reach the settings go to the graphic settings and enable fullscreen
4. save settings and exit everything go back to gaming mode !

you can face some slow perfomance in some parts of the map ( not a big problem ), Just one action requires on the touchscreen ( to open the messages but also be done with joystick, just need to open main menu ).