I don't put much credence in the brs calculator. Amounts needed vary by tank ,organics load,etc. Many use 1/2 cup of bituminous or lignite carbon per 50 gallons of system water changed monthly or 1/3 cup of rox8. I use the rox ;it's the acknowledged best in the industry and has very low dust. Carbon removes organics and increases water clarity,so staring low and working up is important to avoid sudden changes to your tank particularly light intensity increases related to clearer water. Rinsing carbon before use is important.
GFO removes inorganic phosphate, excess metals and silicate. It can exhuast qickly in a tank with high phosphate. It's helpful to measure phospahte when using it,ime. Dropping PO4 to 0.1 or so can go fast without noticeable negative effect on corals,ime . After that for lower ranges a slow drop is better.
GFO may precipitate some calcium carbonate but the effects on alkalinity should be small. The PO4 being removed is also alkalinity but not the type corals use well when it's excessive.
Both gfo and gac are excellent tools for keeping a reef.
Coraline likes good alkalinity and magnesium levels. Green algaes are limited at .03ppm PO4 or less; coraline and rhodophta( red algaes) are not.