In terms of where it is used, here is a related tweet by Filippo Valsorda:

https://twitter.com/FiloSottile/status/1270115515378384897

For scale, this GnuTLS vulnerability is considerably worse than Heartbleed. If you use Linux distributions with GNU tendencies, you might want to check your dependency trees.

(He is a cryptographer on the Go project, who also happened to win the CloudFlare Heartbleed Challenge).

As he says, that thread is a good starting point for potentially vulnerable uses.

Some sample quotes from that thread:

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The good news is that this is a server-side issue

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For obvious reasons, systemd ships a custom http server with client auth via GnuTLS.

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On Fedora "dnf repoquery --whatrequires gnutls" lists: Samba NetworkManager pacemaker qemu / libvirt wget rdesktop tigervnc gnupg

Apache mod_gnutls ...