You can turn on a balance checking feature in your checkout that enables your customers to visit your Gift Up! drop-in checkout and enter their gift card code and we'll response with their remaining balance and expiry date (if applicable).
Here's how you turn it on: Head over to Design > Checkout and turn on the option to allow your customers to check their balance.
Once that option is enabled, you'll see a "Check a gift card balance" link in your checkout footer.
Customers can then enter their gift card code and we'll response with a screen that shows them their balance/expiry.