Please change from "Commons:GNU Free Documentation License, version 1.2" (the default value, used by Commons) to "GNU Free Documentation License" so that link on Template:GFDL (which transcludes it indirectly via {{int:Wm-license-gfdl-1.2-and-later}}) goes to GNU Free Documentation License (the license) and not Commons:GNU Free Documentation License, version 1.2 (a deleted page).
Topic on Project:Village Pump
I've updated it. Actually, I've created that page, but it doesn't update the link in other languages, because this message apparently is provided by an extension, and all languages have its localized link. Looks like it needs to be changed somewhere else...
The message comes from Extension:WikimediaMessages
Looks like someone would need to file a bug report, then
I created a redirect commons:GNU Free Documentation License, version 1.2
The redirect is wrong. It should point to commons:Commons:GNU Free Documentation License, version 1.2 instead.
This works around the problem, not solves it.
There's an open task for this: phab:T250593