@Pppery:: Thanks for the ping!
To clarify the situation a bit...I think Paul Gu's wiki page on the MediaWiki skins he authored answers it the best: "GuMax now has THREE main branches, one is dedicated to horizontal navigation menu layout, drop down menu layout, and vertical navigation menu layout." Sadly Paul's wiki is not exactly in the most usable shape right now, though the download links etc. do work, if you can find them; the Internet Archive is very helpful in this regard.
The skin this page is about, GuMaxDD, is the "drop down menu layout" variant of the GuMax skin written by Paul Gu. There was no public version control repo for this skin, or any of the skins, so I created a git repo, imported the unzipped versions of the source code downloaded from Paul's site into this repo, and this repo was later on imported to WMF gerrit as mediawiki/skins/GuMaxDD.
I don't know about the other two GuMax variants (vertical/horizontal navigation menu layouts), but I presume that one of them is indeed the original Skin:GuMax; those two are currently unmaintained. I've fixed GuMaxDD and tested it against MediaWiki 1.35 and added new stuff nowadays considered standard, like Extension:Echo compatibility, so the GuMaxDD skin is certainly very much functional and maintained, much like the other extensions and skins I('ve ended up) maintain(ing).
On the topic of README files: I'm not a huge fan of them, generally speaking. All too often they end up becoming unmaintained, outdated, useless clutter. A wiki page is easier to edit and more editable by a wider community, whereas README changes etc. have to go through the normal code review process (by which I mean "bitrot to death in gerrit if the author isn't willing to poke a bunch of people all the time and then some"). That's why the SocialProfile extension, for example, has a rather basic README that mostly points to the MW.org info page.
Hope this clarifies the situation, and please feel free to ping me again should you have any further questions!