If you have a timed exam in Gradescope, you will need to grant extensions for an assignment directly in Gradescope. Extensions can be for more time, due date, or late due date.
To set extensions, click on the "Extensions" link on the left side of the screen for you Gradescope course. Then click on the "Add an extension" button.
Then you will get a pop-up window that lets you select the student and the kind of extension that applies to them:
If you do this at the course level, it will apply to all assignments for the course.