ssh -v user@host
you can see
... debug1: Next authentication method: gssapi-keyex debug1: No valid Key exchange context debug1: Next authentication method: gssapi-with-micthan try
ssh -o GSSAPIAuthentication=no -l user host
or change /etc/ssh/sshd_config
GSSAPIAuthentication no LookupClientHostnames no VerifyReverseMapping no