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 $150 per year, you can create a suggested gift card amount of $150 and make the description something like "A full years subscription of my awesome content!"
Navigate to the Marketing tab, select Gift Cards, and click on Settings. On the Settings page, click on Add suggested amount for a new gift card option to appear. Add the amount and a description so your users understand why you are suggesting that value.
You can also drag and drop the gift cards suggestions to change the order of how they appear to your users. Please Save your changes once you're happy with the gift cards.