If you have the legacy free edition of G Suite, upgrade to G Suite Basic 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 Exchange server.
An Exchange role account that Google Calendar uses to access Exchange.
A Google role account that Exchange uses to access Google Calendar.
A single Exchange endpoint for availability information.
Note: We only support a single Exchange endpoint for Google Calendar users querying availability 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 availability 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 availability data at a time. Learn more about GCCIN.