So downloading the game from GOG and attempting to run it straight away simply does not work for me.

The game gives me a grey screen for about 5 seconds, then plays the 3dfx splashscreen.
Then it goes grey again, then it goes into a tiny window on the top left, then it shows the loading screen (the one with the main character with his back turned). Then another grey screen appears, and if i am lucky the next company logo video plays. After that, nothing of a grey screen.
The only way to get out of it from here is to use task manager BLIND as i cant see that.

Stuffing around with nGlide seems to not work.
I've tried adjusting the gamma settings.
I've tried installing the latest version.
I've tried removing the glide dlls from the directory.
I've tried using the sinpatcher.exe
Sometimes the only change i get from everyone elses suggestions so far is that i don't get the 3dfx splash screen and it just locks up with the grey screen right from the get-go...

The only luck i've had is deleting all of the glide and 3dfx dlls. This forces the game to run in software mode.
I get all of the grey screen -> video ->grey screen -> tiny grey screen -> video -> grey screen nonsense still... but at least it gets to the "login" screen (i need to put up with more grey screen nonsense between that and the main menu, but at least i get there)

Once the game gets to the main menu the "typing" effect that plays as the white writing types out on screen is the LOUDEST sound my computer has ever produced. Even at 10% volume on my built-in speakers is loud enough to be heard from the other end of the house.

I can get to the video options menu. And i can even select which renderer i want! But if i select any resolution higher than 800x600 i get an immediate crash (after a good bit of grey screen nonsense), and it is back to watching the game struggle to load for another 60 seconds (it takes that long to get to the "login screen")

When i get into the game, not only does it run in a pseudo windowed mode (basically fullscreen at 800x600, but i get to see the window borders as well), but it also plays REALLY terribly.
Once im on foot, it feels like im riding a unicylce, and not very well.
The game itself runs at the correct speed but my character moves at random, slow speeds (sneaking in thief -> walking in halflife speeds, and everything in between)
The character also moves in semi-random directions (sometimes straight, sometimes ~15 degrees off from the direction im pressing)

I have no idea how to fix this trainwreck. Does anyone have any ideas?

I believe this is a common kind of trouble. You basically go to the launcher or whatever and change the graphics to Default OpenGl instead of 3DFX (which doesn't look great for this game anyway IMO). You may have to rename the game's .exe into Quake2.exe to trick your graphics card to use the right settings when running it. Also fixes the loud sounds once it's in Glide mode.

https://www.gog.com/forum/sin_gold/if_youre_having_trouble_running_sin_try

There should be other similar threads so dig around if this doesn't instantly solve it.

Ok, so what i had to do to get it running was:
1. remove the glide/3dfx dlls from the directory and put them elsewhere
2. run the game so it would default to software mode
3. move the dlls back in
4. select 3dfx opengl from the ingame menu

this fixed my issue.

It is worth noting that i had installed nglide 2.0 prior to trying this (which had no effect at the time) so im not sure if that had anything to do with it

Thanks for the help anyway! Now all i need to do is try and get it to work in widescreen again