When a customer wants to pay using your E Gift card, go to your E G ift cards > Redeem:

If you scroll across to the right, you will see a Reedem button: 

You can redeem their Gift card by pressing this button:

Enter the amount you would like to redeem and submit. If it is not the whole amount of the Gift card then you will still be able to redeem teh rest another time.

Remember, they have already prepaid for this Gift card when it was purchased using Stripe/ Square payment gateway set up to your account.