Once "3rd Party Gradebook Sync" has been enabled, Teachers must do the following:

After connecting a Google account to your Gradelink account, the Google API will prompt you to grant access to your Google Classroom data.

It may be helpful to clarify what data Gradelink will and will not have access to:

Google API DescriptionWhat can Gradelink access?Why is this necessary?
View your Google Classroom classes
The names of all Google Classroom classes associated with a specified Google accountGradelink uses the names of classes to 'match' classes in Google Classroom to classes in Gradelink
View your Google Classroom class rosters
The names of all students enrolled in a specified Google Classroom classGradelink uses names and email addresses to 'match' students in Google Classroom with students in Gradelink
View the email addresses of people in your classes
The email addresses associated with each student in each class
View course work and grades for students in the Google Classroom classes you teach or administerInformation about course work (AKA "assignments") including the title, due date, and the maximum number of pointsGradelink uses this information to recreate Google Classroom course work as "assignments" in Gradelink

Once you click "Allow", the Google API will close and you'll be returned to Gradelink. You should see the Sync Settings page automatically update; when it does, you'll be ready to move on to the next step: setting up valid sync configurations.