Hello!
This is my first contribution to the GPG bug tracker. Please let me know if there is anything missing, and sorry if I missed an existing bug.
I am on macOS using brew's GPG:
```
gpg (GnuPG) 2.3.4
libgcrypt 1.10.0
```
GPG won't let me export a secret ssh key:
```
$ gpg --export-secret-ssh-key 17477E54BB15F155AB66CA72E8BC9929304AEAED
gpg: key "17477E54BB15F155AB66CA72E8BC9929304AEAED" not found: Not a Keygrip
```
But it lets me export the same key as a public key:
```
$ gpg --export-ssh-key 17477E54BB15F155AB66CA72E8BC9929304AEAED
ssh-rsa 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 openpgp:0x632A040F
```
Steps to reproduce:
- Generate a new key
- gpg --expert --edit-key <KEYID>
- addkey
- RSA (add your own capabilities)
- Enable only the A (Authenticate) capability
- Try to export it.
It could be that I am doing something wrong here, but the "not found" error does not help me understand it. Obviously the key exists if I am able to export the public version of it.