In the GSSAPI (Kerberos) Authentication section, the URL for connecting is currently:
client = mongoc_client_new ("mongodb://mongodbuser%40EXAMPLE.COM@example.com/?authMechanism=GSSAPI");
It could be confusing for the KDC domain and MongoDB server to be the same. Perhaps this is better:
client = mongoc_client_new ("mongodb://mongodbuser%40EXAMPLE.COM@mongo-server.example.com/?authMechanism=GSSAPI");
Also, the kinit example seems to have run two CLI lines together:
$ kinit mongodbuser@EXAMPLE.COMmongodbuser@EXAMPLE.COM's Password: