A gradebook entry can have a portfolio attached to it. Doing this allows graders for a course to access specific portfolio entries, and grade them inline with an assessment form, thereby populating the entry in the gradebook.
You need to be a staff:admin, staff:pcoor associated with the course, or a course director in order to access and edit the course gradebook assessments.
Access a course gradebook through the course page or Admin>Manage Gradebook.
Click the pencil icon in the Edit column for the assessment you want to attach to a portfolio to; this will take you to the edit assessment page.
Scroll down to the Assessment Portfolio heading and click 'Attach Portfolio'.
Select the appropriate portfolio and click 'Attach Selected Portfolio'.
If no Portfolios appear for you to select from, make sure you are working in the correct curriculum period and that the affiliated cohort has an active Portfolio.
Additional information about using the gradebook in general is available in the Gradebook help section.