You can use your GMX Organizer with every application that supports the CalDAV standard or .ics format like Outlook, Thunderbird or Mac OS calendar.
The GMX Organizer can be synchronized with all calendar apps that support the CalDAV standard. GMX offers this option as an additional service and has no influence on whether and at what price applications with CalDAV are available for your device or system.
You can transfer appointments from a calendar application to the GMX Organizer if the calendar application supports exports in the "iCalendar" (.ics) format.
The CalDAV address is https://caldav.gmx.com/.