Borghese gallery tour is listed on Day 16 of the tour. Day 16 of our tour is a Monday when the gallery is closed. Does the itinerary change for Sunday and Monday so that the Borghese gallery is included?
For such a specific question about a tour, you probably need to contact the Rick Steves company, rather than the volunteers of the Forum - there are contact links in the blue bar at the bottom of the page.
I did speak with someone in the Rick Steves tour office. Was able to find out that day 15 and 16 get reversed. Tour of Borghese gallery is on Sunday around 2:30 or 3 then on Monday morning there is the walking tour of the forum, colleseum and pantheon.