I read a great article from an experienced GT keeper who successfully raised many juveniles to glorious adulthood. His claim is that the white poo is bacterial mulm from the digestive tract. After treating for parasites as a precaution, it is advised to avoid foods high in starch like corn, rice, etc. and stick to high quality vegetable foods like HBH that are largely comprised of peas, alfalfa, etc. and only have a trace of grains or none at all if possible. This will not only clean the tract, but discourage bacterial growth.
I had a GT long ago and tried every dewormer I could find, and then resorted to harder meds. No dice. He just shrank and shrank until I gave up and returned him. Now I know what I would have done.