Basically the automated refinery from SC2. Here either set it to drain the gold every few seconds via triggers (you can probably make it take upkeep into account), or make it a haunted mine and create invisible, unselectable, 0-food gatherer units that die as soon as the mine does. For all intents and purposes, the gold is gathering itself.
Other possibilities: flying gatherers, gatherers that grab gold in huge chunks (say 5000 instead of 100) but take a lot longer, having no gatherers but the main building channels resources...