Does your app allow buyer to input a custom amount for a gift card ?

According to Shopify, customers cannot input a custom amount for a gift card through your online storefront. 

If a customer wishes to purchase a gift card for a value not available in your store, you can create a draft order for the custom amount instead. Once the customer pays the invoice, you can generate a gift card for that amount and send it to them.

I hope this clarifies your concern.