i dont think any of us here have been able to figure it out, as far as i know, we have only been able to figure out what the red light in the dash is for that comes on when you flip the flipper switch.
Modern cars test all the warning lights when you first turn the car on to make sure they work and the bulbs are not burned out. The GT is an old car, as are all the Opels in the US, and it is from a time before this was a standard feature of every car. The switch is there just to test the light on the dash. On a Kadett it is also the warning light, as they do not have another light on the dash. All it is supposed to do is light up the light to let you know the bulb is good.