an example snippet of how to add a custom field to the Gift Cards by WooCommerce extension

wc-gc-add-custom-field.php

<?php
/*
Below, is an example snippet of how to add a "Recipient Name" field to the Gift Cards by WooCommerce extension

Adding a new field in this form is a technically demanding task, as it must:

- be displayed in the single product page,
- be validated when the Add to Cart button is clicked,
- be included in the Gift Cards Importer/Exporter and;
- be included as a placeholder in WooCommerce > Settings > Emails > Gift Card received.

*/

// 1. Add a name field.
add_filter( 'woocommerce_gc_form_fields', 'sw_gc_add_giftcard_to_name_field' );
function sw_gc_add_giftcard_to_name_field( $form_fields ) {
	$form_fields[ 'wc_gc_giftcard_to_name' ] = 'To (Name)';
	return $form_fields;
}
// 2. Print field html.
add_filter( 'woocommerce_gc_form_fields_html', 'sw_gc_add_giftcard_to_name_field_html', 15 );
function sw_gc_add_giftcard_to_name_field_html( $product ) {
	$value = isset( $_REQUEST[ 'wc_gc_giftcard_to_name' ] ) ? sanitize_text_field( wp_unslash( wptexturize( $_REQUEST[ 'wc_gc_giftcard_to_name' ] ) ) ) : '';
	?>
	<div class="wc_gc_field wc_gc_giftcard_to_name">
		<label for="wc_gc_giftcard_to_name"><?php esc_html_e( 'To (Name)', 'woocommerce-gift-cards' ); ?></label>
		<input type="text" name="wc_gc_giftcard_to_name" placeholder="<?php echo sprintf( esc_attr__( 'Enter recipient\'s name', 'woocommerce-gift-cards' ), wc_gc_get_emails_delimiter() ); ?>" value="<?php echo $value; ?>"/>
	</div>
	<?php
}
// 3. Validate custom field value (Optional)
add_action( 'woocommerce_gc_validate_form_fields', 'sw_gc_validate_giftcard_to_name_field' );
function sw_gc_validate_giftcard_to_name_field( $configuration ) {
	if ( empty( $configuration[ 'wc_gc_giftcard_to_name' ] ) ) {
		throw new Exception( 'Invalid Giftcard To Name' );
	}
}
// 4. Include in export CSV.
add_filter( 'woocommerce_gc_giftcards_export_default_columns', 'sw_gc_export_add_giftcard_to_name_field' );
function sw_gc_export_add_giftcard_to_name_field( $columns ) {
	$columns[ 'giftcard_to_name' ] = __( 'To Name', 'woocommerce-gift-cards' );
	return $columns;
}
add_filter( 'woocommerce_gc_giftcards_export_column_recipient_name', 'sw_gc_export_process_giftcard_to_name_field', 10, 2 );
function sw_gc_export_process_giftcard_to_name_field( $value, $giftcard ) {
	if ( ! $giftcard->get_order_id() || ! $giftcard->get_order_item_id() ) {
		return $value;
	}
	try {
		$order_item = new WC_Order_Item_Product( $giftcard->get_order_item_id() );
		if ( ! $order_item->get_id() ) {
			return $value;
		}
		$value = $order_item->get_meta( 'wc_gc_giftcard_to_name', true );
	} catch ( Exception $e ) {
		// Do nothing...
	}
	return $value;
}


// 5. Display the new Email PlaceHolder in the email tooltips in WooCommerce > Settings > Emails > Gift Card received
// Filter is available from version 1.7.x
add_filter( 'woocommerce_gc_email_placeholders', 'sw_gc_email_placeholders', 10 );
function sw_gc_email_placeholders( $placeholders ) {

	$placeholders[] = 'giftcard_to_name';

	return $placeholders;
}

// 6. Hooks into the dynamic filter woocommerce_gc_email_placeholder_`placeholder_name`_value and replaces
// placeholder_name with whatever value we want. In the snippet below, it replaces {giftcard_to_name} with the
// order item meta wc_gc_giftcard_to_name
// Filter is available from version 1.7.x
add_filter( 'woocommerce_gc_email_placeholder_giftcard_to_name_value', 'sw_gc_email_placeholders_giftcard_to_name_field', 10, 2 );
function sw_gc_email_placeholders_giftcard_to_name_field( $value, $giftcard ) {
	if ( ! $giftcard->get_order_id() || ! $giftcard->get_order_item_id() ) {
		return $value;
	}

	try {
		$order_item = new WC_Order_Item_Product( $giftcard->get_order_item_id() );
		if ( ! $order_item->get_id() ) {
			return $value;
		}
		$meta = $order_item->get_meta( 'wc_gc_giftcard_to_name', true );
		if ( ! empty( $meta ) ) {
			$value = $meta;
		}
	} catch ( Exception $e ) {
		// Do nothing...
		return $value;
	}
	return $value;
}