It probably means “version of the source code used to build the gold master”.

Some places have (or had) a business process of escrowing both the release and the source used to build it. Escrowing just the source used to build the release can require significantly less storage than escrowing the whole version control system. It also avoids the problem “we have the entire revision history, but we aren’t sure which commit was used to build these binaries”

If you lose everything-a colleague told me the story of a company whose offices were in WTC, luckily all the staff got out alive on 9/11, but they forgot to make offsite backups of the source code-the source code to the release(s) shipped to customers is most important, because you need it to make patches. The rest of the revision history, while valuable, is less essential.

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...