Can you post a picture of your GSP?
They say you can't kill GSP- but I have. And in my case, I believe it was the light (aquamaxx prism) I was using when I first started.
In my opinion, I don't think your 'low' nutrients is 100% at fault for it not growing and spreading. My system was running at Ultra Ultra Low Nutrients for months and months when I upgraded to AI Hydra 26- and in that time the new GSP I had bought at the store took over the rock it came with, traveled across the sand from it's rock island and grew up onto one of my main rocks.
Last month, I accidentally knocked my mp10 dry side and broke it, and had to temporarily replace it with a small, less powerful pump. My flow set up prior to this happening was an mp10 on the opposite side of the tank pointing to the gsp side, and an Aqamai KPS parallel to the mp10. I took out the mp10, shifted the kps to where the mp10 was, and filled in the original kps location with a small ac Hydor pump. I mention this because for about a week, both gsp 'colonies' closed up. They have since acclimated to the flow and are back to normal.
Could it really be lack of flow, or just different flow pattern in general?
In summary, my anecdotal experience is that GSP likes good sps-type flow and proper reef lights.
I'll post a quick video of the GSP below in a few minutes.