In the Authentication Examples, the URL for connecting with GSSAPI could be more clear:
uri = "mongodb://mongodbuser%40EXAMPLE.COM@example.com/?authMechanism=GSSAPI&authMechanismProperties=SERVICE_NAME:myservicename"
As the KDC domain and MongoDB server are the same, it's not easy to see where the user principal ends. Perhaps this is better:
uri = "mongodb://mongodbuser%40EXAMPLE.COM@mongo-server.example.com/?authMechanism=GSSAPI&authMechanismProperties=SERVICE_NAME:myservicename"