That's the upstream source repository, not rendered documentation, but
there doesn't seem to be anything better (the
http://developer.gnome.org/gdl/ suggested in the documentation does not
exist; I have reported this as
https://gitlab.gnome.org/GNOME/gdl/-/issues/11 ) and it is common to use
the repository URL in this case (about half of *all* Debian's Homepage
fields point to https://github.com/...).
