Gold as a colour in art is almost never a pigment. It is metal, and the techniques for getting it onto a surface are among the most demanding in the traditional crafts.
Gold leaf is gold beaten to a few ten-thousandths of a millimetre, thin enough to be translucent, and it must be laid on a prepared ground rather than painted. In the standard European method the ground is bole, a fine red clay, laid over gesso; the leaf is floated onto it and then burnished with a stone until it takes a mirror finish. The red bole shows faintly through the gold and warms it, which is why worn medieval gilding reveals red beneath.
Shell gold is powdered gold in a binder, painted with a brush, used for fine lines and highlights where leaf cannot go. Mosaic gold, tin sulphide, and various yellow lakes over tin leaf were the cheap imitations, and workshop contracts sometimes specify that real gold is to be used precisely because the substitutes existed.
In the mosaic tradition of the eastern Mediterranean, gold leaf was sandwiched between two layers of glass to make a tessera, and the cubes were set at slightly varying angles so that the surface flickers as the viewer moves through the building. That is a deliberate optical effect and it does not photograph.
Every symbolic claim about gold as uncreated light was made about a material behaving in these specific ways in a specific kind of room, usually lit by candles or by a shaft of daylight rather than evenly.