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