Bill,
I don't think I have ever seen GSP floss. Why don't you just double up GSP thread for your paraloop and furled hackle? You don't need a lot of strength for this application so there are lots of other material that could be used. One of my favorite for the paraloop hackle is Flex-Floss as after wrapping and compressing the hackle, the length can easily be adjusted to fit the fly.

A tip on paraloop hackle tying - A lot of tiers have difficulty tying in the hackle loop at the head and neatly completing the fly. Try this - place the base of the loop and the hackle tie-in at the point of the rear of the head. Now go ahead and complete the head with whipfinish and move the thread to the rear point where you desire the back of the loop to be. Place a half hitch there to lock the thread at that point (makes it easier later in the operation). Now go ahead and wrap and compress your hackle loop as normal and then tie it down at the rear point. If you use thread matching your body or thorax the tie down will be almost unnoticeable and the head of the fly will be clean and neat.

Regards,
Jim Cramer