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, 2016, or Exchange Online (Office 365®).
Note: Exchange 2010 has some additional setup requirements. Learn more
Inbound and outbound internet connectivity for Exchange Web Services (EWS).
An SSL certificate (issued by a trusted public root certificate authority) for the Exchange server.
An Exchange role account Google Calendar uses to access Exchange. Learn more
A Google role account Exchange uses to access Google Calendar. Learn more
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 need to establish a multi-forest trust between Active Directory® forests. This allows the single Exchange endpoint used by Google Calendar to retrieve availability information across all forests.
Calendar Interop and other Calendar products
If you're using Calendar Interop, you can't use Google Calendar Connector for IBM® Notes® (GCCIN). Google Calendar supports only one external source of availability information at a time. Learn more about GCCIN.