When you create a calendar event in Outlook and invite someone with a Gmail address, that invitee will get an email with a .ics file attached. This event is automatically added to the recipient's calendar, with 2-way sync enabled.
The same works in the opposite direction: Creating a Google calendar event and inviting someone with an outlook address will auto-import a calendar event in that person's Outlook calendar (no email intervention was needed).
Am I correct in assuming that the .ics file itself is not enough for this behavior and that both parties need to implement their own calendar sync server?