The quiz has its own settings for whether the grade for that quiz is visible. The gradebook setting is synchronised to the quiz one.
Given that is the case, the gradebook should not offer the option to manually show/hide the grade, and that was fixed in Moodle 2.2.5.
This bug https://tracker.moodle.org/browse/MDL-18301 explains the details quite well, if you want to wade through it.
Even so, the current situation is not ideal.