A gradebook entry can have a portfolio attached to it. Doing this allows graders for a course to access specific portfolio artifacts, and grade them inline with an assessment form, thereby populating the entry in the gradebook.
Access a course gradebook through the course page or Admin>Manage Gradebook.
Click the pencil icon beside the assessment you want to attach to a portfolio to; this will take you to the edit assessment page.
Scroll down to Assessment ePortfolio and click 'Attach ePortfolio'.
Select the appropriate portfolio and click 'Attach Selected ePortfolio'.
If no ePortfolios appear for you to select from, make sure you are working in the correct curriculum period and that the affiliated cohort has an active ePortfolio.
Additional information about using the gradebook is available in the Gradebook help section.