Google Assignments need to be created from the Assignments link in the Canvas course. If you create it as a new assignment from a Module, it does not work well.
Tip: If you want to see a video tutorial first, scroll to the bottom of this page.
1) From Assignments, click + Assignment.
2) Choose External Tool, and then Find Google Assignments (LTI 1.3)
3) Again, select Google Assignments (LTI 1.3)
4) Verify that your account shows, and then select Continue
5) Google Assignments should be presented. You can either Attach an existing Google doc, or create something new. You can choose to grade in Google's native environment, or choose Canvas Speedgrader. Finally, Create the assignment.
6) You will be returned to the Configure External Tool screen. There are really no instructions but you must click Select.
7) You then have an additional opportunity to complete or edit the assignment settings such as due-date, points, description etc. When ready, click Save or Save & Publish.
8) You should now see your new Google Assignment in the Assignments link from your Canvas course.
9) Now, if you would like to add this Assignment to one of your Modules, you go to the Module and Add an item by clicking +
10) Choose Assignment from the drop-down and pick your Google Assignment as shown below. When ready, click Add Item.
More Resources:
Google Assignments (Canvas Resource)
Use Google Assignments with Canvas (Google Resource)