Since it took some time to find out how to properly launch it on steamdeck (especially due to the crash due to lower than 1080p screen), here is my installation method. I have played only a few areas, essentially the tutorial.

I hope this helps anyone trying to run Fallout London on Steamdeck or any other linux desktop distro.
Note: It looks lengthy, but half of it is just the installation method as per GOG and developers own instruction.

I used Bottles to install the offline installers in a prefix. But the fixes for crashing and adding to gamemode could work similarly with heroic or lutris.
When mentioned below <WINEPREFIX>, it looks like this when using Bottles: `/home/user/.var/app/com.usebottles.bottles/data/bottles/bottles/Fallout-Collection`

## Install

- Create a bottle in Bottles
(A bottle is a wine prefix, nothing more, nothing less)
- Run the following with "Run Executable":
- Install GOG installer of Fallout 4 GOTY in the bottle
- Install GOG installer of Fallout London in the same bottle
- Run the Fallout London installer (<WINEPREFIX>/drive_c/GOG Games/Fallout London/installer.exe) and point it during installation to the Fallout 4 GOTY directory

## Post-install per developer advise

- Change settings using the Fallout 4 Launcher
-- Use "Run Executable" and run Fallout4Launcher.exe (<WINEPREFIX>/drive_c/GOG Games/Fallout 4 GOTY/Fallout4Launcher.exe)
-- Make sure to set it to Borderless, as this should prevent the unable to move bug when exiting terminals.
-- The mod is quite heavy, especially smoke/lighting, so set it relatively low
- Make sure to start Fallout 4 at least once until you reach main menu.
- Run the Fallout London installer.exe again and hit play to test it
(We will keep using the installer to launch the game, again per developer advise)

## Fix/prevent crashing issue

- It crashes on lower than 1080p monitors, so go to `<WINEPREFIX>/drive_c/users/steamuser/Documents/My Games/Fallout4/Fallout4prefs.ini\` and set:
-- iSize H to 800
-- iSize W to 1280

## Adding the game to gamemode

Note that I set it to launch it in the original Bottles prefix, where also F4 GOTY is located. Also the installer sets registry values in the Bottles prefix, which will be missing when running over proton.

- Target: `/home/user/.var/app/com.usebottles.bottles/data/bottles/runners/soda-9.0-1/bin/wine "<WINEPREFIX>/drive_c/GOG Games/Fallout London/installer.exe"`
- Start in: `"<WINEPREFIX>/drive_c/GOG Games/Fallout London/"`
- Launch options: `WINEPREFIX="<WINEPREFIX>" %command%`

Hey there, thanks a million for this guide, I was trying to get this set up on my deck and it was driving me nuts. I have just one issue: I can't get adding the game to my Steam Library to work. I set everything up as you described, with <WINEPREFIX> pointing to the folder I installed my bottle in, but the game just starts loading momentarily, and then it drops me right back to my library. Any idea what's the issue here? (I had to install my Bottle on my SD card due to space constraints on my internal HD, could that be a problem)?

The Game launches fine straight out of Bottles though, but I have to go to Desktop mode in order to launch it from there (strangely enough, it won't start if I just add bottles to the Steam Deck and use that as secondary launcher either, it just sticks to the Bottles UI there).

Nevermind, found a solution: I installed BoilR through flatpack and asked it to automatically import all Bottles games into my Steam library. So that's sorted then.

Glad you managed to fix it.

The three entries are quite finicky, it may be a double-quote missing, which means any spaces in your path will break the command that is running.

BoilR crashes for me (it crashes on Fedora, and I have Bazzite, which is based on Fedora), so I could not test this, but good to know it works as that is way more convenient than manually forcing Steam to ignore proton :)