GOG version of the game launched via Heroic. At first, the game was totally unplayable - constant crashes, crackling audio and insanly fast map scrolling. All of those problems vanished after one change. When you open Fantasy General window in Heroic (in desktop mode) you have 3 options = 1. Launch Options; 2. Fantasy General; 3. Graphic Mode Setup. Choose the last one and than in "Basic Settings" set "Graphic Mode" to OpenGL. Save Settings. Problem solved. I also changed game resolution to 1280x720 ("Advanced Settings", "Window Resolution") in orded to force full screen display via Stretch scaling mode from the Steamdeck's Quickmenu.