I am running Win10 and downloaded Carmagedon to pc. I have the game in the program menu but am unabe to locate the downloaded files/folder on the pc. Looking for the guide ect. I allowed the download to auto install to the default location. Thanks.
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found em :)

Open File Explorer and search for Carmageddon.

That should cough up both the installed location of the game and also the location of the downloaded installer files. That assumes you actually downloaded them via the browser or the (unsupported but functioning) Downloader, and didn't instead install through Galaxy.

That also assumes I understood your question correctly. Haha. I probably didn't.

A Galaxy user will likely chime in with info on how that particular installation process works, but I don't think a Galaxy installation saves the installer files to your computer.

If you used Galaxy the downloaded files should be in C:\Program Files (x86)\GOG Galaxy\Games which is the default location. While I think this user was looking for the game folder (not an installer) but it does as long as you download the actual offline installer via MORE -> Backup & Goodies. Works pretty much like the old downloader.