Installing open-ssh in order to use it to connect to Beskow/Tegner, the online described way to install openssh with the gsskex flavour wasn't working in my case for macOS Catalina. An alternative to macport is to use home-brew:

    brew tap rdp/homebrew-openssh-gssapi
    brew install rdp/homebrew-openssh-gssapi/openssh-patched --with-gssapi-support

which worked just fine.