To import email from Google, please follow the instructions provided here for 'Setting up OAuth 2.0'. You should choose 'Internal' for the User Type - you probably don't want the application accessible outside your organization.
Next, you need to enable the Gmail API. In the Google Cloud admin console at console.cloud.google.com, select the 'Enabled APIs & services' link. The screen should look similar to this:
If Gmail API isn't listed, use the 'ENABLE APIS AND SERVICES' link to add it.
Finally, select the 'Credentials' link in the Google Cloud admin console. Use the 'CREATE CREDENTIALS' link to create an OAuth 2.0 Client ID. The credentials you create should be for 'Desktop Apps'.
On creating the client, you will see a confirmation dialog similar to this one:
Copy and paste the 'Client ID' and 'Client secret' fields to the corresponding fields in the Email Import QSX configuration dialog.
Now, click 'Connect' on the Email Import QSX configuration dialog to acquire an OAuth token. This will launch a browser window and ask for your credentials. Upon login, the 'Test' section and button can be used to test the configuration.