Found between the Square and Compasses, within a star, it is a common in Freemasonry. There are a lot of explanations what the letter stands for. Needless to say that this differs per lodge. In a very Christian lodge, the G may be explained as “God”, in a secular lodge “geometry” may be preferred. There are more (possible) explanations though. In some old ‘high degrees’, the G. stands for “Gomez” or it is the first letter of the first word spoken by Adam.