gnupg - Are GPG and SSH keys interchangable?

I want to generate an RSA key in GPG and use it in SSH login. Is this even possible? If so, how?

edit: see @wwerner's answer, I didn't try it but it seems to be the current solution (as of 2018)

Answer

I know this is an old post, but for people like me stumbling over this:

It is now (since gpg 2.1) possible to simply extract ssh keys directly using gpg: gpg --export-ssh-key !.

The ! mark is optional, it makes the primary key exportable and omits checking whether the key is authentication-capable ([CA]).

Details: