It's a good idea to add some suggested gift card amounts that correspond to the lengths of your subscriptions. For example, if you have a subscription that is $100 per year, you can create a suggested gift card amount of $100 and make the description something like "A full years subscription of my awesome content!"
Navigate to Marketing > Gift Cards and click Settings. You can add your suggested gift card amounts here and drag and drop the order you want them to appear for your customers.