Used as a parameter of [curl->set] and CURLOPT_HTTPAUTH to direct curl to use HTTP GSS-Negotiate authentication. The GSS-Negotiate (also known as plain "Negotiate") method was designed by Microsoft and is used in their web applications. It is primarily meant as a support for Kerberos5 authentication but may also be used along with other authentication methods. For more information see IETF draft draft-brezak-spnego-http-04.txt.

You need to build libcurl with a suitable GSS-API library for this to work.