Extract or recreate original SSH private key added to GPG2 keyring

gpg2ssh

$ gpg2 --list-secret-keys --keyid-format short
 /home/user/.gnupg/pubring.kbx
 -------------------------------
 sec   rsa4096/53C2371D 2020-06-18 [SC]
       47009EB0BA7E95211A217130D77BCDBD8340250A
 uid         [ultimate] User Name <[email protected]>
 ssb   rsa4096/53AF00DS 2020-06-18 [E]
 ssb   rsa4096/CA7F00DS 2021-02-18 [A]

# Notice I'm not using the primary key, but instead the first subkey with 
# authentication capability "[A]".
#
# Also important to append the "!" to the key ID given to gpg2. From the
# manpage:
#   | By specifying the key to export using a key ID or a fingerprint 
#   | suffixed with an exclamation mark (!), a specific subkey or the 
#   | primary key can be exported.
#
# The openpgp2ssh utility is part of the MonkeySphere project. Most apt-
# based distros can install it via 'apt install monkeysphere'.

$ gpg2 --export-secret-keys CA7F00D5! | openpgp2ssh CA7F00D5 > ~/.ssh/gpg.id_rsa
 Please enter the passphrase to export the OpenPGP secret key:
 "User Name <[email protected]>"
 4096-bit RSA key, ID D77BCDBD8340250A,
 created 2020-06-18.
 Passphrase: 

# Verify output is identical to original, SSH-formatted RSA private key.

$ file ~/.ssh/gpg.id_rsa
 .ssh/gpg.id_rsa: PEM RSA private key
$ diff ~/.ssh/id_rsa ~/.ssh/gpg.id_rsa
$