CalSync mirrors events between a Google calendar and a Microsoft one, so time booked in either place shows as busy in both. It works with personal gmail.com and outlook.com accounts as well as Google Workspace and Microsoft 365.
What it does
Mirrors events in one direction, or both, between any two calendars you connect
Marks every event it creates, and will only ever modify or delete events carrying that mark
Picks up a change as soon as the provider notifies us — usually within a minute — using each provider's change notifications
Reconciles nightly, so a missed notification corrects itself
What it does not do
It never touches an event it did not create
It does not read or store the contents of your email
It does not sell, share, or advertise against your data