korell: To bring this thread up to date, then, we'll list what has been covered so far.
Okay, so which nGlide version does the GOG installation of Interstate '76 come with? I'm sure it uses nGlide, so there would be an nGlide configurator and DLL files within the game installation folder somewhere, probably the root of the installation alongside the game executable. If it is an old version, I'd replace it myself, though GOG sometimes provide older versions as they couldn't get it working consistently reliable with the latest version on their test rigs.
Then, as you have nGlide installed on your machine itself, change the settings for it first, and then go to the nGlide configurator within the game folder and set the settings there (older versions of nGlide have slightly different options, and as settings are stored in the registry they are shared between all nGlide versions, which has caused issues in the past due to conflicting values. But yes, vsync should be turned on in nGlide.
Personally, I don't use nGlide as an installation, I just extract the files to the games I want to use it on, as different versions with different games is then easily possible and it makes sure that you can't get conflicts between the nGlide DLLs in the Windows system folders and the nGlide DLLs in the game folder (whether they would ever conflict anyway I'm not sure, but this way they can't as there aren't any in the Windows system folders).
Next, use Nvidia Inspector to create a profile for the I76 executable, and then set the frame rate limit for that executable to something around 29-30 FPS. You can also set the vsync option here (try forced on, or 1/2 refresh if you are on a 60 Hz monitor, or 1/4 refresh if you are on a 120 Hz monitor. The Nvidia Control Panel probably sets similar options if you use adaptive v-sync, too.
I got it all working by removing the I'76's glide dll files and using my nglide installation I already had along with the other stuff. I think the weapon physics are working correctly too. I can now see the flamethrower coming from my car but just a tiny bit, is that how it was in the original?