These two documents provide further information about GSS-API:
Generic Security Service Application Program Interface document (http://www.ietf.org/rfc/rfc2743.txt) provides a conceptual overview of GSS-API.
Generic Security Service API Version 2: C-Bindings document (http://www.ietf.org/rfc/rfc2744.txt) discusses the specifics of the C-language-based GSS-API.