GPG encrypt and decrypt with ssh-agent

I use ssh regularly and have ssh-agent set up.

How can I use ssh keys to gpg-encrypt a file?

Edit: It seems that this is impossible. Why? ssh can encrypt traffic, so why not also files?

No, SSH keys are not PGP keys and serve different purpose .
You must have PGP keys in GnuPG keyring.
But - you can convert GPG keys to SSH keys (using gpgkey2ssh) and gpg-agent can take care of SSH keys (using --enable-ssh option). I believe no such tool exists for converting SSH keys to GPG keys and cannot exist.

It seems that this is impossible. Why? ssh can encrypt traffic, so why not also files?

SSH can encrypt, just not in a GPG compatible manner.

Per https://unix.stackexchange.com/questions/27005/encrypting-file-only-with-ssh-priv-key:

openssl enc -aes-256-cbc -in my.pdf -out mydata.enc 

and

openssl enc -aes-256-cbc -d -in mydata.enc -out mydecrypted.pdf

should do the trick.