While the app does not currently send automatic reminders, If a gift has not been redeemed by the recipient, you can send a reminder by sending the redeem a gift email again from within the app.
Open 'Gifts'
Locate the gift/recipient you would like to send a reminder to.
Click the three dots at the end of the row.
Select the 'Resend' option.
An email will be sent to the recipient's email address provided by the customer at the time of order. the email will be the same one sent back when the order was placed with the gift message attached. An example of the email can be found below.