Items in the grade book are available for sync with [sis]. However, sections are not in the sync and individuals are not available for sync. Assignments can be synced with sections, but the grade book overall does not currently support syncing by section or by individual.
The Instructure Product Team is working on implementing a better solution to address this feedback, which would allow instructors to assign due and availability dates to multiple variations of sections and individuals, enabling differentiation within an assignment.
Currently, to sync by section, you must use the assignment-level sync functionality, not the overall gradebook sync.