I would like to reference my GradeBook from last year. Am I able to retrieve it?
If you created a final backup at the end of the previous year, technically you could restore it, however, THAT would overwrite your current GradeBook! This is not recommended. What you can view is all Final Grades for a student in all subjects in SIRS or SchoolLogic. If you haven't been trained in SIRS/SchoolLogic, consult your building's administrator for same.
If you are wanting to set up your GradeBook using the same Units, Categories and Tasks as the previous year, you can import a Template. But for that to work, you would have had to save your class, last year, as a Template to be able to use it this year. Such a Template wouldn't include Students or their scores; only Setup info you created.