Once in a while, official Debian package upload lag behind. Here's how to build your own. Tested on Ubuntu 20.04.

Run these with root privileges.

  # apt-get build-dep gnucap
  # apt install dpkg-dev fakeroot

These better run without root privileges.

  $ git clone https://salsa.debian.org/electronics-team/Gnucap/gnucap
  $ cd gnucap
  $ dpkg-buildpackage -rfakeroot --no-sign -d
  $ ls ../*gnucap*deb

The -d flag was necessary because debhelper-compat (=13) is not available for Ubuntu 20.04. Here, debhelper only includes debhelper-compat (<=12), it appears to be sufficient.
Then install (as root).

  # dpkg -i ../*gnucap*.deb