gift-card-save-settings.php

<?php
add_action( 'woocommerce_process_product_meta', 'save_gift_card_options_field' );

function save_gift_card_options_field( $post_id ) {

	$enable_gift_card = isset( $_POST['_enable_gift_card'] ) ? 'yes' : 'no';
	update_post_meta( $post_id, '_enable_gift_card', $enable_gift_card );

	if ( isset( $_POST['_gift_card_price'] ) ) :
		update_post_meta( $post_id, '_gift_card_price', sanitize_text_field( $_POST['_gift_card_price'] ) );
	endif;
}