We have done only a one-off ingestion of GNU package releases (from https://ftp.gnu.org/) back in 2015.
We should periodically ingest new GNU package releases, automatic the listing process.
ftp.gnu is now available only via HTTP (and no longer via FTP), but an up-to-date directory listing is available at https://ftp.gnu.org/tree.json.gz (thanks Ludovic Courtès for the heads up on this).