If you have the legacy free edition of Google Apps, upgrade to G Suite to get this feature.
To use Calendar Interop, you need:
Microsoft® Exchange 2010, 2013, or 2016, or Office 365TM (recommended).
Note: Exchange 2010 has some additional setup requirements.
Inbound and outbound Internet connectivity for Exchange Web Services (EWS).
An SSL certificate (issued by a trusted public Internet root Certificate Authority) for the EWS server.
An Exchange role account that Google Calendar will use to access Exchange.
A Google role account that Exchange will use to access Google Calendar.
A single Exchange endpoint for free/busy information.
Note: We only support a single Exchange endpoint for Google Calendar users querying free/busy information of Exchange users. For a single Exchange organization, you can typically use any Exchange server. For multiple Exchange organizations, you might need to establish a multi-forest trust between Active Directory® forests. This will enable the single Exchange endpoint used by Google Calendar to retrieve free/busy information across all forests.
Calendar Interop and other Calendar tools
If you are using Calendar Interop, you can't use Google Calendar Connector for IBM® Notes® (GCCIN). Google Calendar supports only one external source of free/busy data at a time. Learn more about GCCIN.