Tarballs for newer releases are available from the package registry and new and old release are also available on download.gnome.org.
To get the source code, use
$ git clone https://gitlab.gnome.org/GNOME/gucharmap
Gucharmap ships with a copy of the unicode data files it needs to build, but you can also provide an external (absolute) path to the unicode data files to the -Ducd_path=... argument when running meson setup.