Randy's currently on vacation.
As long as you are happy with your current nitrate level and it is being maintained by your current dose rate, I see no reason to change it.
You can stop using GFO and see if your phosphate level starts to rise. If so you will need to continue using GFO or possibly reduce your fish food phosphate added (lower phosphate foods).
If after you stop using GFO and your phosphate level remains where you want it, then I see no need to continue using GFO.