I guess game developers don't consider GOG customers worth serving. It takes a while for the manual patches, of course...but Galaxy ought to update the same day as Steam.

Yeah, I'm still waiting for the patch on GOG as well :( Hopefully, they will push it on GOG the next days as well. I totally agree with your opinion as well. If you buy it on other stores except of Steam, you should be treated the same way.

The FSR 2.0 update is only available for the DX12 version on the game, according to a Chernobylite dev posting in the Steam forums.
https://steamcommunity.com/app/1016800/discussions/0/5154944131318194192/

DX12 was for some reason removed from the game on GOG with the last Chernobylite update.

I asked on the Steam forums about DX12, and a dev replied. You have to add
-dx12
in the Chernobylite launch parameters in GOG Galaxy for the game to launch with DX12. When that is done, FSR 2.0 is available in the advanced graphics options (FSR 2.0 is only available with DX12).

How to add the -dx12 parameter:
- In the game's page in your library in Galaxy, next to the Play button is a pull-down menu with additional options. Choose "Manage Installation > Configure"
- Select the Features tab
- Under "Launch parameters" check the box for "Custom executables / arguments"
- Click "Add another executable / arguments"
- Browse to the Chernobylite installation folder, and select ChernobyliteGame.exe
- add the argument -dx12
- if you want this to be the default configuration, click the box for "Default executable." Clicking the Play button in Galaxy will then launch the game using this custom configuration. If you don't want this to be the default configuration, skip this step.
- click OK

If you didn't set DX12 to be the default, you can launch the game by clicking that pull-down menu next to the Play button. You will now have an option for "Additional executables." The option you created should be visible there. Click it to launch the game using DX12.

I tested this and it works; Chernobylite launches using DX12, and FSR 2.0 is available in Advanced Video Settings.

You're my man! I did what you described in your post and I can finally select FSR 2.0. Thanks. :D