Start by going to your Event Page. After this, go to Event Settings and scroll down to the bottom where you will see Advanced Settings. Make sure Gift Card Event is turned on to say YES (see image below.)
After this is done, navigate to the Event Setup step. This is where you can set up different increments for your gift cards by clicking Add Level.
Next, on the Customize Ticket step, you can edit and design what you want your gift card to look like. The gift card level and ID will be included by default.
After someone has purchased a gift card, it will accumulate under their contact record as well as under Event Details with a 16 digit code they can use on a future event.