GFO is supposed to be run in a fluidised state so the GFO particles have maximum contact time with the water run thru them. That is why GFO reactors like the TLF, Kent, etc are clear so you can see and adjust the water flow thru them to keep the GFO moving within the reactor, but not moving so much that it goes to the top and out of the reactor.
I don't know how you'd adjust for this in an opaque cannister filter.
I assume the recommendation for a fluidized state is because the GFO only binds phosphate on its surface, and not within pores like GAC, which you pack so it does not move in a reactor.