You can cancel/abort any task that is in progress (including a migration) by accessing the Tasks menu, hovering your cursor over the progress symbol, and clicking Cancel Task.
You can also cancel your task from the task's loading screen using the Cancel Task button at the top-right corner of the screen.
Cancelling any task will stop it from progressing further, however it will not undo any progress that has been made up to the point of cancellation. You can view the progress made up to the point of cancellation on your results screen or Migration Report.
Note: In the case of a Migration, ShareGate Desktop may have already sent calls to the servers that it will attempt to finish before it can stop the task. If you notice that the migration is not cancelling, you can force quit the app to ensure that the migration does not progress further.
What if my task is still running once I cancel it?
Note: If you are running ShareGate Desktop 11.0.3 or older - when you see a green progress bar at the top of your migration report, there is no way to cancel the task. 
Your cancel request may see a delay when Microsoft 365 is importing items from Azure. This is something that can occur when using Insane Mode with a Microsoft 365 destination.
At this point, ShareGate Desktop pushed all your content to Azure with instructions on how to set it in your sites. Everything is handled by Microsoft 365. The only thing the application does, is wait for a confirmation. Because ShareGate Desktop is sending a request to Azure to cancel the import, it may take longer.
Note: If you see no progress in the green bar over 24 hours, there could be an issue with the queue. You can safely ignore the running task, and re-attempt your operation. We recommend reducing your migration scope, and checking your network to make sure ShareGate Desktop is able to properly communicate with Microsoft 365.