The boot on the GS series II is sprung so that with a gently push the boot will close without futher assistance. However adding a spoiler to the boot lid will cause it to slam due to the extra weight. Luckily the tension can easily be adjusted to allow for this extra weight.
First open the boot and remove the side trim which is attached via the plugs circled in red in the picture below.
Once the trim is pulled back you can see the end of the torsion bar is located in a hole in the metalwork. There are three positions, the default position is towards the front of the vehicle.
Grab hold of the bar, pull inwards then pull the bar back towards the rear of vehicle and locate in one of the other holes.
The picture below shows the bar in the highest tension setting.
Do the same on the other side, test the operation of the boot lid and then replace boot trim.