I have dug a little deeper into the history of that line and found that it was proposed to be gpg-agent but was converged to gnupg-agent because it was installable on more platforms. On Xenial gnupg-agent has the content, on newer platforms it’s just the documentation, with a dependency on gpg-agent.
gpg-agent is only available back through bionic: https://packages.ubuntu.com/focal/gpg-agent
gnupg-agent is available on all versions: https://packages.ubuntu.com/focal/gnupg-agent I believe mostly as a backwards compatibility package it’s labeled “(dummy transitional package)” on the newer platforms. It’s produced from the same source package: Ubuntu – Details of source package gnupg2 in focal
– Sorry, as a new user I can only post two links so I had to quote two of the links.