The letter G is a Masonnic symbol, which is found in the Fellowcraft degree, occupying the centre of the Blazing Star.
Some argue that the letter 'G' stands for GEOMETRY, the basis of the operative craft that gave birth to modern Freemasonry,the science of the Great Architect of the Universe; but such an explanation takes on its true meaning only if you substitute G for the Greek gamma, whose shape is that of a square.