You don't need the official patch, it is already installed in GOG version. Get the unofficial one (trust me, you want it) here instead (CtP2 Apolyton Edition):
http://ctp2files.apolyton.net/source/

The mystery of trying to figure out what 'unofficial patch' Elwin was talking about here. I've been browsing apolyton for 4 days trying to find anything like an 'unofficial patch' and nothing.

It looks like they have gone through some major changes and the mod/unofficial patch is now gone.

The site moved to github

Here is what you need.

https://github.com/civctp2/civctp2/