Stop using the Galaxy launcher and launch them locally.

Nope.

And now it's not running again. Clicking on the starter exe just brings up the blue ring next for the curosr for a few seconds and nothing happens.

Umm.. did you download all the parts before running the exe? (I only see the patches in your screenshot, some even duplicates?)
And you didn't rename the files, right?

Don't use the Galaxy client to install games. (I never have. I unistall the client if it is "automatically" installed as part of a game's installation.)

The error message is because you are trying to execute the installation file and Widows cannot find the other part/s (the Gothic….BIN files that correspond to the executable Gothic….EXE that kicks off the self-extracting installation process) so you have probably (unintentionally) downloaded the files to different directories.

I'm using the Gothic starter exe, the same fucking exe that I managed to successfully start the game from earlier and for some stupid fucking reason it's not working anymore now.

I uninstalled, reinstalled the offline version, did everything exactly the way I got it run earlier and it just doesn't start up anymore, what a fucking travesty.

That's usually called "user error".

Go to your game folder on the site or Galaxy and download ALL the files available in said folder. Locate the downloaded files, check to see you got all of them, then run the installation file. For in-game fixes check the specific forum on Gothic.

Gothic runs fine. Here's how I run it under Windows 10 (i've just re-installed it and did this as I went.) It should also be the same for Galaxy installations just skip part 1:

1) Download it from the non-Galaxy installer.

2) Install the game.

3) Download and apply the fullscreen fix from here (only needed if on Windows 10/8/8.1) otherwise the game will have graphical issues and will not go full screen.

Extract the file anywhere, then run install.bat as an admin.

4) Navigate to Gothic\system\ then right click on GOTHIC.exe and click 'Properties' Click the 'Compatibility' tab then tick the box marked 'Run in 640x480 screen resolution.'

This is needed otherwise the game will just crash on the opening video.

5) Run Gothic from Gothic\system\GOTHIC.exe (the GOG shortcut should also work, or you can just delete that and create a shortcut to GOTHIC.exe)

Have you tried applying the compatibility mode, that I posted above in step 4?

Didn't you say that's only needed for W8/10?

Fullscreen fix is. Compatibility mode should be for 7 as well.

Try this guide:
https://steamcommunity.com/sharedfiles/filedetails/?id=484664831
Leave out the DX11 patch if you're not sure if your graphics hardware supports DirectX 11.

You know what I appreciate the help but I'm fucking done with this. I play games to relax and having to bust my balls for fucking hours troubleshooting is not very relaxing to me, I neither havethe patience nor nerves for this bullshit. I'll just go replay Risen on my Xbox. Fuck PC gaming, fucking pain in the ass.

With Windows you have to ensure yourself that you have all correct drivers installed and updated, legacy components enabled and DirectX 9 installed next to DX12.