G may not be an 'official' abbreviation but it is in fairly common use - esp in atlases / gazeteers. I have just looked up Mexico & Bothnia in my Times Index -Gazeteer (1965), and they both have an entry under G. of !!
Thank you for that Kettledrum. I'm not sure about the abbreviation being in fairly common use, as I said in my post to Sarumite I could not recall a skilled setter using it in any crossword I've done in over 60 years.