"Cryptography in SSH does not necessarily require Public Key Infrastructure (PKI) for authentication. GSSAPI also works. Password also works"
GSSAPI is just an API, not a method.
As for password, it doesn't matter what you call it, password, encryption key or Trevor, you still have the same problem of sending someone encrypted data without them giving you an encryption key someone else could intercept and use as a decypt key. This is the problem public-private key was designed to solve and quantum comms aside its currently the only method.