gift-card-custom-product-add-to-drop-down.php

<?php
add_filter( 'product_type_selector', 'wcpt_add_gift_card_type' );

function wcpt_add_gift_card_type ( $type ) {
	// Key should be exactly the same as in the class product_type
	$type[ 'gift_card' ] = __( 'Gift Card' );
	return $type;
}