Hi,
I'm no expert but i guess that you will not get distance info being fed back to the camera. As for the D/G- type lenses, the common feature is the ability to feedback subject distance to the camera.
Therefore, Nikon's 3D Matrix flash metering will fall back onto "pseudo- 2D" flash metering as the distance of the subject is not taken into consideration. D70 has this 3D Matrix flash metering mode.
For difference between G-type versus all the other types of lenses, G-types have no aperture ring => user has to use the camera to select the aperture. Not a problem if using with D70.
Personally I dun bemoan the loss of the flash distance D/G-function IMHO.