The GSSAPI is an IETF standard that addresses the problem of many similar but incompatible security services in use today.
The GSSAPI SASL Mechanisms provides a way for clients to authentication including using a Kerberos V5 session.
The Generic Security Service Application Program Interface is described in the following RFCs:
RFC 4752, and a description of the exchange between the client and the server (as well as with the Kerberos KDC).
Unlike the GSSAPI, the Kerberos API has not been standardized and various existing implementations may use incompatible APIs. The GSSAPI allows Kerberos implementations to be API compatible.