Gold Bar is an item in Postknight that is used in trading to obtain pieces of equipment when shopping at a Merchant's shop or to interact with love interests.
Gold Bars are highly obscure items when compared to other items, having the inability to be dropped by any enemy from any area or as loot from chests. Their only source is from trading and relationship gifts, therefore they are obtained from Merchant's shop of Valley of Gold which stock up on Gold Bars, while Asteria can provide all three types of bars.
Gold Bars are resident to a small subcategory of items: trade items. That being their only purpose is to be used to trade for different items to aid the player in their adventure. Similar to the lower tiered Copper and Silver Bars, it is used to unlock the equipment pieces and allow the player to grow alongside the difficulty curve that the game naturally presents. The Gold Bar unlocks the fifth tiered sets of equipment.