PSpice Thyristor DC Switch
The current source in your simulation is ideal.  That means it doesn't care if the switch is open or closed.  It will produce very high voltages to make the 100A flow through the gate even if the switch is open.  Best bet is to replace it with a voltage source.  100A is way to much gate current.  Try replacing I1 with a 5V source and making R6 1k.