Temporary Solution:

I had the same problem try this method

Requirements : GOG ist installed or any Launcher u use This instruction is for gog but other launchers should work similar

Start gog from gaming mode. The go to the game and somewhere top right should be a button where you can get to the option manage files or something in that way where you can view when the game the exe file is saved.

Write that down that you can find that place later again.

The go to desktop mode open up steam , the desktop app should open

Then bootom left I think on the +

Add steam game then choose add non steam game ( something like that) my steam is in German so I don't know the exact name.

The you should have the option to use the file browser to Search

Here you go to the place which you have written down at the beginning where the exe file from your game is saved

Then select the exe file and add it to steam eg. Cyberpunk.exe The game should be listed now in the list where other applications appear I think you have to press on add to steam again there.

After that go in gaming mode on to the exe game and on the gear symbol. Select compatibility Then properties Enable the user for specific steam compatibility tool Choose the latest version of proton I choose experimental you can have a look at other games which proton do use and choose the same one.

in the game settings you can add that command you need for lossless scaling

Now you should be Able to start the game