If you allow customers to purchase gift cards with custom amounts, you'll need to create a gift card item with no set value.
Go to Items in the main navigation menu.
Press + New Item.
Under NAME, enter “Gift Card Custom”.
Under CATEGORY, select Gift Cards.
Under TAX CLASS, select No Tax.
Under PRICE, enter 0.
Under SKU, enter giftcard.
Under CUSTOM PRICE, select Enabled.
Scroll down and press Save.