Look up the spec sheet on greensand per cuft and I'm pretty sure you'll see higher th n5 gpm for a 1.5cuft greensand filter.
Also, there are two of us, maybe three, that says more is needed if the filter is going to operate as it should over time, than the 5 gpm the seller is calling for. That's based on actual experience troubling shooting the stuff over many years. Personally I wouldn't sell greensand (Pyrolox too) filters and didn't since like 1992 or so.
Plus the fact that the constant SFR of the 1.5cuft filter seems to be too low for the household's peak demand flow rate.