Under the Java connection examples found at http://mongodb.github.io/mongo-java-driver/3.3/driver/reference/connecting/authenticating/

See the GSSAPI examples, the connection string is quoted as follows:

MongoClientURI uri = new MongoClientURI("mongodb://username%40MYREALM.com@host1/?authMechanism=GSSAPI");

The realm component must be all upper case, and I would suggest making the realm name consistent with the previous example (quoted above this string), as follows:

MongoClientURI uri = new MongoClientURI("mongodb://user1%40MYREALM.ME@host1/?authMechanism=GSSAPI");