If you are running them together they should be mixed. You can't tumble the carbon and GFO together because the carbon will be ground up. Mixing will help prevent the GFO from clumping with the lower flow rate.
Nam, check your nitrate levels. Running gfo depletes phosphates but does not remove nitrates. If you can't lower nitrates via water changes, I will consider a form of carbon dosing.
I used peroxide to rid the tank of a huge infestation a few months ago. It worked surprisingly well and grows back much slower than it once did. It grows in the sand about 2' a week in areas of lower flow. It also grows on some of the rocks right at the sandbed and behind corals for the most part. I feel it is under control for the most part but I just wish I could get rid of it completely. I have had dozens of reef tanks over the years and have never had an issue with GHA until now.