Okay, I keep reading that I need this file for eduke32, where the hell is it?! I look in the gog directory, and lo and behold, there's no DUKE3D.GRP. There's the RTS file I see, but I see no DUKE3D.GRP file, so um, HEEEEEELP!!!!

Yet there is no file marked DUKE3D.GRP that I can find, the closest I come to it is a "microsoft program group". now if that's the damn file I'm looking for all well and dandy. that's find, but don't mark it as solved if you haven't told me what the file will be named as, or the file type will be stated as. Telling me "it must be there" is NOT SOLVING THE PROBLEM!

the file will be DUKE3D.GRP
it should be in:
C:\Program Files\GOG.com\Duke Nukem 3D
on 64-bit versions of Windows it be in:
C:\Program Files(x86)\GOG.com\Duke Nukem 3D

The closest I come, is this: "DUKE3D" with "microsoft program group" as its descriptor, no "GRP" after DUKE3D

Probably you have 'common files extension' set to hidden.
You can change that setting in Folder Options.

That's probably it, right-click it and go to properties and you should see sometihng that says "type of file" and it should show you the extension there in parentheses, for example "microsoft program group (.grp)".

If it has a .grp then that's the file.

I really recommend doing what Thiev says and unhiding file extensions. Not only will it help you find this particular file but it's also a useful security feature to show file extensions as some viruses may be executable files showing a common icon like a Word document icon, so when you think it'll open a document it actually runs the virus code.

Personally, I think file extensions should always be shown by default, I really see no need to have them hidden.

I agree, but the one reason to keep them hidden is that when re-naming files it can be easy to accidentally change the file extension if you're not careful. Could lead to some headaches especially if the user is not used to dealing with file extensions.

Yes, I suppose so, but I must say that I am surprised that Microsoft have never just made it so that when renaming a file only the name gets highlighted for overwriting (and giving the user the option to highlight or delete the extension if desired) rather than selecting the full name and extension.

But then I've always worked with file extensions.

Yeah, you'd think that would make sense, right? Anyway, in response to the original question, one can either check the file properties to see extensions or set windows to always display them. I figured checking the properties in this case was the simpler solution, unless the user wanted to be able to see extensions for other reasons in the future.