Gradebook has 2 required uses.

1. Attendance
2. Posting of Midterms and final grades

So right now you do not need to even log in to it.  All your assignments are and should be completed in Canvas.  And Canvas only.  Assignments will push to Gradebook.  I have always had people ask about assignments that are not completed "electronically" or in Canvas.  Not a problem.  Put scores in Canvas - Stay out of Gradebook!
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I am seeing teachers put content in Canvas as an assignment, then go to GSD Gradebook and adding the same assignment to enter scores.  You're working to hard and adding too much data! Just record the scores in Canvas.  Either in the Grades screen above or in Speed Grader below.

You can now add Google Hangouts Meet DIRECTLY in to your Canvas class. You can add it anywhere but if you want students "live" it may be best add to your calendar if you want students to know when your it is.  In my example below I added it to a page, but you can add it to most editable items in Canvas. It is just a code/link for them to click.
UPDATE on 3/30 I do not see the external tool icon in the calendar editing box.  I would make a page with YOUR hangout info on to store... Then make a calendar item and copy and paste YOUR hangouts info into the text box.