My first reaction to what you're describing is that this sounds as if it's exactly the same behavior as we have and expect for all assignments in a course. For homework and exam/gatewayquiz assignments alike, grades are available to the students before the answer date. I think in general this is desirable, certainly for homework and in many cases probably also for exam/gatewayquiz assignments. At this instant I don't know of any way to change that behavior for any assignments, including gatewayquiz assignments
My second reaction is that you could be seeing a bug in the "show scores" setting for a test. If you have this set to "No," grades should not be reported to the students on the grades page. This is the behavior that I'm seeing on our system (WeBWorK and PG 2.7): with "Show Scores on Finished Assignments" set to "No," students do not see their grade on the grades page. I haven't checked the behavior of the setting "Only after set answer date," but the code in the Grades module makes me think that should also be working correctly.
Is that not what you are seeing?