This was a fun little project. I wanted to use GnuPG keys to authenticate to my Raspberry Pi but I wanted to store my private keys on my Yubikey. The configuration documented is using gpg4win 3.1.11 and a Raspberry Pi using OpenSSH version OpenSSH_7.4p1
Now that I have GnuPG keys this documentation is what I did to add them to a new (MacOS) client and configure the MacOS GnuPG client to use the authentication key with SSH authentication.
If you are looking for any of my public keys I will slowly start adding them to this page.