That's correct, but the magnitude value isn't dBi either. It's the current generated by the planar wave. So unless you know what current would be generated in a isotropic receiver or a resonant dipole (with the appropriate "source" resistance), then you can't calculate dBd.I believe most uhf/vhf antenna comparisons use dBd = dBi minus 2.1 .
However, the comparative gain is still useful. i.e. you can determine that the response at channel 53 is 9dB less than channel 25, you can assume that you're going to have problems picking up CITY-DT. Or if you tweak your SBGH model and you lose 1dB on ch25, while gaining 8dB on ch53, that's a good thing if you're in the GTA.