The story
Gnus is a rewrite of GNUS by Masanobu Umeda, which ceased to be developed in 1992. In autumn 1994, Lars Magne Ingebrigtsen started the rewrite under the name (ding) which is a recursive acronym for ding is not Gnus, intending to produce a version for which the interface and configuration would work almost exactly the same, but the internals would be completely revamped and improved. The new version proved to be popular and has undergone constant expansion and enhancement. Ingebrigtsen is also the lead programmer of eww.
Versions
In general, users receive Gnus bundled with their copy of GNU Emacs and only need to worry about version numbers if they want to upgrade to newer versions themselves instead of receiving updates through Emacs or their operating system's packaging system.
The following versions have been released:
Gnus 5 ((ding) Gnus) – November 1995
Gnus 5.1 rebranded 5.0.13 bundled with GNU Emacs 19.30/19.31
Gnus 5.2 (September Gnus) – May 1996
Gnus 5.3, a rebranded 5.2.38 bundled with GNU Emacs 19.32 and all later versions of GNU Emacs 19.x
Gnus 5.4 (Red Gnus) – January 25, 1997
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Gnus 5.5, a rebranded 5.4 bundled with GNU Emacs 20.1 (September 17, 1997) and also included in XEmacs 20.4
Gnus 5.6 (Quassia Gnus) – March 8, 1998
Gnus 5.8 (Pterodactyl Gnus) – December 3, 1999
Gnus 5.10 (Oort Gnus) – May 1, 2003
Gnus 5.11, a rebranded 5.10 bundled with GNU Emacs 22.1 (June 2, 2007). 5.10/5.11 development from the "Oort" development branch wrapped up around 2008. Some 5.11 versions, such as that packaged by Ubuntu Linux as "5.11+v0.10", are actually based on the later "No Gnus" development branch.
Gnus 5.13 (No Gnus) - bundled with GNU Emacs 23.1 (July 29, 2009)
The "No Gnus" development branch began January 4, 2004, and wrapped up in early 2012, with version 0.19.
Gnus 5.14, Ma Gnus (or 真 Gnus), is the current development version, v0.7 first released in May 2013.
The odd minor version numbers, like 5.3 and 5.5 are for the Gnus versions bundled with GNU Emacs. The even version numbers are the unbundled releases. So for example, Gnus 5.5 is similar to Gnus 5.4, but bundled with Emacs 20.1.
Development is done using "named versions", whose first letters run backwards in the alphabet; "No Gnus" v0.19 was released in early 2012, and development transitioned to "Ma Gnus". No named version ever reaches 1.0, instead when it is considered stable enough for general release, it sheds its name and gets packaged with as simply "Gnus <version number>". Entering the "V" command in the Groups buffer of a running copy of Gnus will usually cause it to divulge a version number, but there is no easy way for an end user to know if, for example, "No Gnus 0.9" is older or newer than "Gnus 5.10.8".
After being developed separately for 22 years, the developer of Gnus announced that further development would take place inside Gnu Emacs' git tree. A side effect of this change is that support for XEmacs and older versions of Gnu Emacs will be dropped.
From Wikipedia’s article on Gnus, shared under CC BY-SA 4.0.4