Where do I forge gold bars?
According to what I found, it's done in a Forge or Foundry, but neither has gold bars, or whatever, in the list.
You need the build the Melting House (in the Precious Items Production part of the Refined Resources toolbar) to make gold guilders from the gold ore. It's the only use for the gold if I remember right (other than straight up trading the ore).

The silver on the other hand can be transformed either into coins at the melting house, or into bars at the forge/foundry, because it is used further on to make silverwares used for fancy homewares required to build the more advanced houses.