gnome-common is deprecated and has been replaced by autoconf-archive; it should not be used in new projects. Existing projects may continue to use it until they choose to port (there’s no rush!).
gnome-common was a collection of utilities for the GNOME project. Almost all GNOME modules should use this.
Graphical applications should probably include help and use yelp.m4.
Git: git clone https://gitlab.gnome.org/GNOME/gnome-common.git