I think you are confusing different things toby. GW can always be called up, that's the same one as on the FUEL PRED page and the SD; in other words: the loadsheet. Dan was referring to a call-up of the FAC computed GW, which is basically what the aircraft thinks it weighs, as computed by the FAC using INIT B CG, alpha (AoA), IAS.
So to wrap it up:
FM computed speeds are called by GW
FAC computed speeds are called by GWFK