Couple questions,
What size is that tank?
How long has it been cycled?
Do you do weekly water changes of at least 10% min.?
Any filtration setup for softy coral toxins? Charcoal etc? Something else or none?
GSP does like good lighting, so moderate lighting at least as well as good water quality & water flow that isn't linear, but moves everything back & forth like a wave maker does, or multiple small power heads etc.
This many soft corals causes a lot of toxin warfare more then hard corals so water changes & charcoal filtering within reason not too much would really help.
After all this is addressed then lighting would be at the bottom of the list if all else failed.
I do remember my GSP being my ( canary in a cage) tank indicator in the beginning & would be very sensitive & close up if anything was not spot on before anything else i had
would start to decline, so its telling you something is not within min. marine parameters for soft coral in a tank, hopefully the list above will help to locate the issue.
Good luck tweaking is part of the fun!