ProtonDB - Half_Life this site is best way to check if game will work on deck, or do some tinkering to getting work

Go to game, there is gear icon next to launch. That’s steam game settings.

There is button that show list with resolutions with name default. Try change to native or res you have in monitor.

where you changing res in game settings or steam game settings? I had same problem and change resoluton to native in steam setting fix problem for me. You can also try … button and check profil for game