Gold Bars are most commonly made by smelting Gold Ore.
Gold Bars can be smelted at a forge by a miner with at least 155 mining skill. Smelting consumes one Gold Ore per bar.
Alchemists can also transmute Iron Bars into Gold Bars using their Philosopher's Stone and the Transmute: Iron to Gold transmutation. The transmute requires an alchemy skill of 225.
Solid chests will occasionally (5-8%) have Gold Bars in them.
Gold Bar as an Ingredient
Gold Bar as a Quest Objective