GPG subkeys marked with the "authenticate" capability can be used for public
key authentication with SSH. This is done using gpg-agent which, using the
--enable-ssh-support option, can implement the agent protocol used by SSH.
A working gpg2 setup is required. It may be possible to use gpg 1.4 but with gpg-agent compiled from gpg2. If you are using OS X 10.9 (Mavericks) then you may find the instructions [here][1] useful.