Here's a graphical example:
The red arrow represents the direction your bow is pointing at. A torpedo that follows the same course as you, straight from your bow, is obviously a zero Gyro Angle torpedo, and it will follow the red arrow in its course -same as your ship's course.
Then the moving needle in the dial, highlighted in green represents what Gyro Angle must your torpedo do in order to hit the target that is shown in the upper dial.
Hence, when the needle matches the red arrow, that means that the torpedo must not turn at all to hit the target, i.e. it will be a Zero GA shot, and the moment of shooting has arrived.
Is it all understandable?
