There are multiple ways for your customers to fill their wallet with your currency. They can earn it by participating in a WooRewards Loyalty Program or you can set cashback rules in products pages.
Go to WooCommerce → Products and edit the product for which you want to give gems when sold. Select the Virtual Wallet tab on the left.
Enter the amount of credit customers will earn when they buy this product. You can use this field to give wallet cashback when customers buy a regular product or you can use it to sell your currency to your customers.