You need to allocate enough gold to buy a gun. Guns are weapons of the old world, so you can't just buy them with paper money. Guns have to be bought with gold coins and slabs and bricks.
You need this gun so you can steal some jewels (じゅう)! Why didn't you just use your gold to buy the jewels, you ask? Because! That's not how we did things in olden days. When you want jewels you buy a gun with gold and rob a jewelry store!