fortnightly-notification-via-GP-Notification-Scheduler.php

// filter recurringly weekly notification to instead schedule as fortnightly
add_filter( 'gpns_schedule_timestamp', function ( $timestamp, $notification, $entry, $is_recurring, $current_time ) {

    // properties for a recurring weekly notification
    $form_id = 1;
    $notification_id = '63595d7aecb08';

	if ( (int) $entry['form_id'] !== $form_id || $notification['id'] !== $notification_id || ! $is_recurring ) {
		return $timestamp;
	}

    return strtotime( "+1 week", $timestamp );

}, 10, 5 );