Will a Nikon g mount fit a Nikon d80 camera?

Yes, it will fit. However, you will still need to see if the lens you have has or does not have features like autofocus or CPU connectors that will carry all the info that your camera needs... If it is a Nikon Lens it will work fine. So will others like Sigma, Tamron and Tokina. Carl Zeiss may not give you all the functions but will work in manual mode.