Google Assignments has 3 problems: 1) the grades don't transfer to canvas 2) you cannot separate students into groups or sections when grading, so in my case I am stuck grading 70 papers at the same time rather than 35 today and 35 tomorrow. 3) Three teachers have reported that when they use Group Assignments with "Grade each student individually," students are reporting that they are getting the message that other students have already turned in their assignment.
A fellow teacher figured out one of the problems above. If you want grades to be transferred automatically from Google Assignments to Canvas, you have to set Gradebook to automatically post grades. It's in the gear under Grading Policies in the gradebook.
I too am having the same issue, I grade assignments in google and grades are not uploading into canvas. I tried the suggestion to Automatically post grades, but mine was already on that, so annoying it's not working.