Thanks to the effords of @gabops and the Flatpak team Nagstamon is now available at Flathub: https://flathub.org/apps/details/de.ifw_dresden.nagstamon.

Nagstamon at Flathub

Just install via:

flatpak install flathub de.ifw_dresden.nagstamon

Run it via:

flatpak run de.ifw_dresden.nagstamon