The MOD doesn't require GALAXY to install or run, right? Why in the store page here says that you need to use the Galaxy client?

By only mentioning the galaxy client as a way to run it, you are hiding the truth.
Nothing wrong with mentioning the galaxy client way of installing, but at least make it clear it is not the only way. Otherwise it's very close to lying.

On the dev's website, they make things clearer: https://fallout4london.com/release/

Quote: "You can either use the GOG Galaxy App to download our installer OR
You can manually download Fallout: London from the GOG website DRM free.
Instructions for installation are included in the installer."

Indeed, the instructions should be more like:

If you own Fallout 4: Game of the Year Edition on GOG, since our version is directly compatible with Fallout: London, you will only have to follow these easy steps:
1. Install the English version of Fallout 4: GOTY Edition. If you are using GOG GALAXY, disable your cloud saves.
2. Get Fallout London in our store for free.
3. Install Fallout London.
4. Launch Fallout London. If you are using GOG GALAXY, run Fallout: London from its own game card. Do not use the Fallout 4 game card. If you're not using GOG GALAXY, make sure you're using the f4se_loader.exe file.
5. Follow the launcher’s instructions, and enjoy!

update:Installing the latest F4SE from Nexus and then reinstalling London seems to have fixed my issues. I can launch it from the London Card in GOG now, and am no longer seeing the error below. Seems to be playing correctly now, for the moment at least..


I've installed it this way within GOG Galaxy, with new, clean installs of both the main game and the mod, and it does not run when I press play in the Fallout: London game card. Nothing happens at all when I attempt to run it from there. The only way it runs is from the Fallout 4: GOTY card. Running it from the F4: GOTY card, then it plays without issue until I hit the first conversation after the train, then it freezes with a large error message that says:

F4SE Status: Fail
F4SE is not running. Make sure that F4SE is installed and you have launched the game with (the message is cut off here)

while the conversation keeps running in a loop up to the point where I should give a response, but no response options are available.

That doesn't fix the crashes. That's how I played it from the beginning.

This game has always had pretty bad performance but London is much worse for me, even with lowered graphics settings.
It feels so bad

Done all this still crashes, no error message or anything, game just closes.

Edit: Got the game working after deleting the fallout 4 folder in C:/users/*username*/AppData/Local. Note that AppData is a hidden folder.

I start the game using the Fallout: London icon on my desktop. When on the screen where I can select a new game, there is a message at the bottom saying that "Mods are currently loaded. Achievements are currently disabled".
When clicking "New" I get a black screen with a green icon to the lower Right showing it's loading. When tabbing out of the game (because I'm tired of waiting) it goes to a screen where I can only select Settings and Exit. When selecting Exit, it goes back to the screen where I can select a new game. This I can continue doing in a loop.

My GOG Fallout 4 GOTY and Fallout: London are installed in the same GOG folder on one separate SSD and my My Documents folder is installed on a separate SSD.

System:

Win 10 Pro fully updated.
CPU: i9 9900K
Ram: 32 GB DDR4 3200
Monitor: 2560*1440 144 Hz
Graphics: RTX 4070 Ti

Running at factory setting

I've played mods for years but never one set up like this. Was I supposed to install the mod into my Fallout 4 folder instead of it's own folder like I did? The "f4se_loader.exe" file in the Fallout London folder does nothing. The same file in the Fallout 4 folder loads the London game but then I can't walk 20 feet before it crashes.

So far the Game Fallout London runs very well, except two not repeatable CTDs. But this also happens in vanilla game. I am also playing with some mods!. You must have English FO4-version 1.10.163.0 and all DLCs, which I downloaded from GOG. f4se is delivered by London install.

I keep my FO4 (loves it with my mod selection) and made a parallel FO-London. Becaus of the stupid folder constrution (plugin.txt) two users are needed! (In early games, save- and config-folder were in game folders.- Which was an intelligent thinking!)

FIRST: Mind the gap!

The intallation is unusual.
- When you run the "setup_fallout_london_2.31_revision2_(64bit)_(75054)" from GOG, it will install in a folder "Fallout London" but it should be renamed "Fallout London Setup" or "Fallout London Installer" because it is not the game but a repository.
- After this, you get a shortcut Starter icon "Fallout London". When starting the first time, it installs the mod into your "Fallout 4" folder. It checks language and version.
- Now you can start the game with this shortcut. Your FO4 is gone with all its mods***, because the plugin.txt ist replaced.
- The starter shortcut every time checks the installation and plugin.txt and keeps the London inallation "clean".

If you want use mods in Fallout London and/or keep your Fallout 4, follow the steps below.

Two Fallout 4 installations
- user1: "Fallout 4" (German)
- user2: "Fallout 4" (English) renamed after London-install to "Fallout London"

- before you install the second FO4 you must rename your first to "Fallout 4 bak" or similar. Later you can undo this step.

After installing Fallolut London in the second FO4, you must not use the Installer-Starter! anymore! Never-No-Never!
Best: delete it!

Make shortcuts:

user1
Fallout 4: "D:\Games\Fallout 4\f4se_loader.exe" -forcesteamloader

user2
Fallout London: "D:\Games\Fallout London\f4se_loader.exe" -forcesteamloader

replace "D:\Games\" with your path!
Remind the gap!

*** can not imagine that someone played FO4 without mods e.g. without at least "Unofficial Fallout 4 Patch". With the right mods FO4 is realy fun. Without it is crappy, buggy impertinence.

Don't do this. This will break your game by deleting the London worldspace esm.