A real use for gold for which no other metal could be substituted

Generally, the value of gold is based on psychology and psychiatry and not on economics of physics. It is a good conductor but there are other good conductors which are not expensive. Likewise it can be used for dental fillings but there are cheaper and better materials for dental fillings.

But there is one thing you could use gold for for which no other known material could come close to doing as good a job. Soft, totally inert, and half again more dense than lead, gold would be the absolute ideal metal for waterfowl shot. You could kill ducks and geese all day long with ordinary 2.75" shells and #7 shot. You could actually take the old heirloom guns without the Magnum sized chambers down off the wall and use them again. You would, of course, have to alloy it with something harder to let it withstand the acceleration without deforming.