I followed the official instructions here but generated brainpool512 keys instead of RSA. Everything went fine, and I imported the key on a Debian 9 system. Now, when I wanted to export/convert the key to openssh format I get the following output from gpg 2.1.18:
gpg: export as ssh key failed: Unknown elliptic curve
I upgraded gpg to 2.2.10, but the error message remains the same.