Presumably the source code for the gold master - “Gold Master Source Code” was written on the disk itself. The Imgur link is no more, but you can still see a preview image of it in the original Reddit post. Judging from the comments, it also sounds like the OP may have looked through the contents on a live stream and confirmed it was source code.

https://www.reddit.com/r/gamecollecting/comments/640iem/foun...