You will find plans in books, articles, and on the web. Wentworth has an extensive collection of books and scholarly journals, which is where the highest resolution and quality images will be found.
Our library staff created a subject guide to help you. Take a look at the Architecture Subject Guide and choose the drawings and plans tab for more suggestions.
In searching the Avery Index (Harvard AND graduate And Gropius), I found several article citations on the influence of Gropius at Harvard.
Also a search of the online catalog (Gropius Harvard): had several books that should contain the plans that you need.