We should be notified when a resubmission is made for a Google Assignment. Right now, the only way I know to go and check is by having students message me when they've resubmitted something.
If a resubmission is made in the Google Assignments LTI the prompt that generates a notification through Canvas needs to be developed by Google.
We'll move this forward as a request for a new notification in Canvas associated with a Google Assignments LTI resubmission— but if it is prioritized, its completion will be dependent on Google's development efforts.