price by weight

Hi,

You have the rates auto update plugin which comes with HikaShop Business to update the currency rates. It supports several sources including Open Exchange Rates. And Open Exchange Rates supports XAU (gold):
docs.openexchangerates.org/reference/supported-currencies
So, in the System>Currencies menu, you can create a new currency with the code XAU. Then, you can configure your cron task in the HikaShop configuration, and configure the rates auto update plugin via the Joomla plugins manager to connect it to your Open Exchange Rates account, and it will periodically update the rates, including the gold rate.
Open Exchange Rates has different pricing plans based on what you prefer for the refresh delay of the rates. The cheapest is hourly. The most expensive is 5 minutes.