First, start by clicking on your Event Page. Next, go to the Event Settings step. Scroll down to find Financial Information, and then turn the Accepting Gift Card toggle to YES. Next to this, you can also give the Gift Card field a caption. 

Next, go under Event Setup and add your ticket levels and gift card increments. Once this is done, you can go to the Gift Cards tab and check off which gift cards you would like to accept for the event. 

After this is all implemented into your event page, users will be able to enter their gift card code and use it on their event purchase. The image below shows what this will look like when someone uses a gift card for an event.