Sorry, I had missed this.

The format is

    [gpg]
      format = ssh

    [user]
      signingKey = key::SSH-PUBKEY

In my case, since I have an ssh-ed25519 key, it is

    [user]
      signingKey = key::ssh-ed25519 AAAAC…

You can pretty much copy the `.pub` key and paste it after `key::` (instructions like the ones at https://calebhearth.com/sign-git-with-ssh will show this without the `key::` prefix; according to the latest git documentation, the non-prefixed SSH key format is not preferred, as the `signingKey` value can be either a raw SSH public key, a GPG key ID, or a path to the signing key).

One thing of note that I haven’t checked to see if it matters. I have the matching `.pub` key in `~/.ssh`, but I don’t think that is making a difference.

I do remember having some struggles getting it sorted out at the very beginning, but I didn’t take very good notes of what different things I tried.