Gold Bars would first appear in Luigi's Mansion as an item that could be picked up by opening treasure chests found throughout the mansion. Each Gold Bar would be worth 100,000G each.
They would return in Paper Mario: The Thousand-Year Door and Super Paper Mario to sell for coins. When Mario buys 100 items at any store, he is rewarded with a Gold Bar. If he buys 300 items, then he will receive three Gold Bars. Deepdown Depot would also sell single Gold Bars for 110 coins and Gold Bar x3 for 350 coins. A single Gold Bar can be sold back for 100 coins, while the Gold Bar x3 can be sold for only 300 coins.
They are also set to appear in the upcoming Luigi's Mansion: Dark Moon game.