Phosphate is generally not something you put on auto pilot.
The GFO is very effective at adsorbing PO4. If you check the outflow of the reactor, you can tell when the GFO is used up. The outflow PO4 concentration starts out being near zero PO4 when the GFO is fresh to being the same as that of the aquarium water when it peters out. Flow through the reactor is adjusted to prevent the GFO becoming a settled mud cake, but not so fast to cause the particles to tumble into dust. The size of the reactor determines how often you need to change the spent GFO. You can’t really screw this up. Manufacturer recommendation is a good starting point for size. If you are limited in space or cash, go smaller. There are no rules about how often to change the GFO. You need to measure the PO4 of the outflow.