This is a feature that is not enabled per standard. But today anybody can ask for it and have it activated free of charge.
To add vouchers you first need to log into Backoffice.
Then you must create a payment method. Choose Configuration -> Payment Methods.
Click Create Payment Method.
Let's create the new payment method.
Type should always be Voucher. Write in a name, e.g. Gift Cards, and create the payment method. Next step is creating a product group and product for gift cards.
Go to Products -> Product Groups.
Click Create Product Group.
Make sure the type is Credit Items. Let's call the group Gift Cards in this case. Click Create Product Group.
Now we need to make a product under the group we just made.
Go to Products -> Products.
Create a new product.
Enter a name and choose the correct Product group. Check Open Pricing and change the Type to Voucher. Choose the payment method we created earlier and period of validity doesn't matter, set it to Unlimited.
Click Create Product when you are done.
The last thing to do is to create a button for the gift card.