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
​
826010690104-uj22ep02jimb905l1kikbvpeuo7j8ck6.apps.googleusercontent.com

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.