General Post Office - GPO - Common term for the administrative entity of the postal system.
Geprüft - (German) Censored or checked to be genuine.
Gregorian Calendar - A calendar in general use today, introduced in 1582 by Pope Gregory XIII as a revision of the Julian calendar. Adopted in Great Britain and the American colonies in 1752, marked by the suppression of 11 days after 1700, 12 days after 1800 and 13 days after 1900, and having leap years in every year divisible by four, with the restriction that centesimal years are leap years only when divisible by 400.
Gum - The adhesive substance found on the back of stamps when sold to the public, to enable the stamp to be stuck to the envelope etc.
Gutter - The space between one unit and the next, whether a stamp, pane, or sheet. The term is generally applied to the wide space between panes.