According to a B&H listing:
G-type lenses are fully compatible with the F6, F5, F100, N80, N75, N65, N55, D1 or D2-series and D100 (or any newer model) cameras.
I think G lenses don't have an external apeture ring, my 12-24mm f/4G ED-IF DX doesn't.
the G lenses are designed for cameras that can change the aperture using dials and knobs...the D lenses have the aperture ring and can go on manual and fully automatic nikon cameras.