Each Client/Company of ZeoCRM would require to create an paid Account in Google.
They would create an Application in Google Console to point to this Onboarding Domain
Post approval, Client/Company will generate an offline token to manage Google Calendar
Google Calendar API
ZeoCRM would require this offline token to Sync this Google Calendar with the one available in ZeoCRM
Please note all your contacts, appointments and other information will only not be stored in ZeoCRM
ZeoCRM will use the Google Calendar API and this offline Token to read information and render it on the fly in the ZeoCRM
Gmail API
ZeoCRM using Google API to read gmail powered API, to render the same in the ZeoCRM email client. This is to ensure that one gets onestop solution to manage all action using one Application
ZeoCRM does not stores or read your email content. It is read from the Gmail API and rendered in your email client.