From:
John Leach's Encyclopedia of Postal Authorities :

G

Gabon - Germany
Ghadames - Guyana

Gabon - Germany

Gabon
Gabon (French Colony)
Galapagos Islands
Galata
Galicia
Gambia
Gaspé
Gaza (Egyptian Occupation)
Gaza (Indian UN Force)
Gdansk
GEA
General Gouvernement
General Postal Union
Geneva
Georgia
Georgia (pre-Soviet)
German Cameroun
German Colonies
German Commands
German East Africa
German East Africa (Belgian Occupation)
German East Africa (British Occupation)
German East Africa (Portuguese Occupation)
German Levant
German New Guinea
German Ninth Army Post
German Occupation Issues (WW1)
German Occupation Issues (WW2)
German Post Offices Abroad
German Post Offices in the Turkish Empire
German Samoa
German South West Africa
German States
German Togo
Germany
Germany (Allied Occupation)
Germany (Belgian Occupation)
Germany (Imperial)
Germany (Third Reich)
Germany (Weimar Republic)

Ghadames - Guyana

Ghadames
Ghana
Gibraltar
Gilbert Islands
Gilbert & Ellice Islands
Goa
Gold Coast
Gorizia
Graham Land (Falkland Island Dependencies)
Granadine Confederation
Grand Liban
Grande Comore
Great Britain
Great Britain (Regional Issues)
Great Comoro
Greater Lebanon
Greece
Greek Occupation Issues
Greek Post Offices in the Turkish Empire
Greenland
Grenada
Grenadines of Grenada
Grenadines of St Vincent
Griqualand West
Grodno
Guadeloupe
Guam
Guanacaste
Guatemala
Guernsey
Guinea
Guinea-Bissau
Gulf of Kotor
Gumultsina
Guyana
Gwalior