register-gift-card-as-custom-product-type-for-wc.php

<?php
add_action( 'plugins_loaded', 'wcpt_register_gift_card_type' );

function wcpt_register_gift_card_type () {

	class WC_Product_Gift_Card extends WC_Product {

		public function __construct( $product ) {
			$this->product_type = 'gift_card'; // name of your custom product type
			parent::__construct( $product );
			// add additional functions here
		}
    	}
}