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 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.