Propel Appointment's deposit feature uses Shopify's draft order feature to create partially discounted orders.
When customers use the deposit feature, they're taken to Shopify's draft order checkout rather than the normal checkout. Unfortunately Shopify's draft order feature does not yet support gift cards as a payment method yet.
This is a limitation in Shopify and not in Propel Appointments as our app does not control the checkout environment. Here's a page from the Shopify support forums explaining the situation:
https://community.shopify.com/c/shopify-discussions/draft-orders-no-codes-allowed/td-p/1257884
In that post, a Shopify employee mentioned a possible workaround, though it's not ideal.
They suggested that customers could call and mention they'd like to use a gift card to pay the draft order, and merchants could manually mark the order as paid. We understand that this is not really workable though, and we wish we had a better solution to offer.