Hello,
I have played Fallout NV a lot but this is my first try to get some mods working.

I installed NVSE and launch the game trough "nvse_loader.exe".
This works fine but GOG Galaxy doesn't notice the game is running, which means I won't get achievements and my play time won't get tracked.

Is there a way to get this working? I like achievements. :)

I found a reddit post which looks promising.
(Sorry, but I can't post links unfortunately...)

So I edited the file "goggame-1454587428.info" and replaced "FalloutNV.exe" and "FalloutNVLauncher.exe" with "nvse_loader.exe".
When I launch the game trough GOG Galaxy it still launches "FalloutNVLauncher.exe" and NVSE is not enabled.

Then I also edited the file "goggame-galaxyFileList.ini" and replaced "F18=FalloutNV.exe" and "F347=FalloutNVLauncher.exe" with "F18=nvse_loader.exe" and "F347=nvse_loader.exe".

This also doesn't work. :(

But then I found a shortcut "Launch Fallout - New Vegas" in the Fallout New Vegas folder.
Right click this file > properties > target
Change "G:\GOG\Fallout New Vegas\FalloutNVLauncher.exe" to "G:\GOG\Fallout New Vegas\nvse_loader.exe" and start the game trough GOG Galaxy. (The path may be different on your computer)

This worked for me.

It still works if I change "goggame-galaxyFileList.ini" and "goggame-1454587428.info" back to the original file. So the only thing you have to do is change the target .exe of the shortcut.

PS: This was supposed to be a question but meanwhile I figured it out by myself so I decided to post it as a little guide anyway. Maybe some of you face the same problem.

Editing the shortcut doesn't work for me, so I just renamed the launcher file and ran "mklink .\FalloutNVLauncher.exe .\nvse_loader.exe".

To edit a game running through GOG Galaxy, you HAVE to quit Galaxy and restart it AFTER you have edited the .info file. It works fine for me. I run many games this way.

You actually earn achievements while playing NVSE they just won't show up ingame, they instead appear on your stats page in the galaxy library.

Ive done similar to what you did but i found it WAY way easier to just rename the files themselves in the new vegas folder rather than messing with GOG's files, but i added an addition to my way of doing it since im running mod organizer, if people want to run with game tracking / overlay and run mod organizer + nvse + maybe an enb you can read how i did it here
https://www.gog.com/forum/fallout_series/galaxy_playtime_tracking_through_mod_organizer_2/post7