Compute gravity field signals due to bathymetry using gravfft

Hello GMT family,
I want to use gravfft to compute gravity field signals due to, a grdcut of GEBCO_2022 (downloaded from There are no outputted signals for the -F options, except for gravity anomaly. Can someone kindly help me out? Thanks very much.
I used the script below in MATLAB:

gmt('grdsample -rp -I1m -R-160/-145/-45/-35')

Geoid = gmt('gravfft -T7000/2700/3300/1035 -Fg -Z9000 -E1'); % no signal (NaN)
North = gmt('gravfft -T7000/2700/3300/1035 -Fn -Z9000 -E1'); % no signal (NaN)
East = gmt('gravfft -T7000/2700/3300/1035 -Fe -Z9000 -E1'); % no signal (NaN)
Grav = gmt('gravfft -T7000/2700/3300/1035 -Ff -Z9000 -E1');
VGG = gmt('gravfft -T7000/2700/3300/1035 -Fv -Z9000 -E1'); % no signal (NaN)

subplot(2,3,1); imagesc(Geoid.x, Geoid.y, flipud(Geoid.z)); colorbar;
subplot(2,3,2); imagesc(North.x, North.y, flipud(North.z)); colorbar;
subplot(2,3,3); imagesc(East.x, East.y, flipud(East.z)); colorbar;
subplot(2,3,4); imagesc(Grav.x, Grav.y, flipud(Grav.z)); colorbar;
subplot(2,3,5); imagesc(VGG.x, VGG.y, flipud(VGG.z)); colorbar;

Not sure exactly what you are trying to compute. You have a piece of seafloor that has depths from over 6600 m up to a few hundred meters. You then say “use that grid as a flexural load on a 7 km plate and various densities”. I cannot imagine anything good will come out of passing a -6600 m load on that plate. I suspect what you actually want is to use the bathymetry as a density contrast between basalt and sea water? If so there is -D. I am less sure about -Z. Hopefully @Joaquim can help us here.

Thank you @pwessel for the response. The -D option in place of -Z gave me what I am looking for.

-Z is, for example, for setting the Moho average depth when computing its gravity effect due to the crust load. The manual has several examples of its usage.