As an admin, login to G-Suite console (https://admin.google.com/u/2/ac/home) - you should see something like this:
Open security settings (hamburger menu - security - settings) - you should see something like this
Click API permissions
You will now see App access control
Click on "Manage Third-Party App Access". You should see something like this:
Select "Configure new app" and click on "OAuth app name or Client ID"
You will see the below screen:
Enter Zapflow application OAuth2 Client ID
Now Click Search. You will see the below screen
Select Zapflow. You will see the below screen:
Click to make the tick mark appear and press Select. You will be taken to the below screen:
Select Trusted and click Configure.
Now everything should be set and you should be able to do your email sync from within Zapflow.