Indeed Lincoln and Roxbury are the only gravel descents. There are also some stretches of level gravel (Quaker St in Lincoln, Upper Plains Rd in Middlebury) and even a mile or so of gravel on the ascent of Lincoln Gap put there no doubt to discourage dilettantes. I did the ride last Sunday on 622-28 tires inflated to 7 bar (100 psi), but I wouldn't recommend it.
Apropos descents: although only two are gravel, all six deserve respect. App Gap in particular has some hairpins that nobody should try to take at speed, and there are straightaways on the Brandon descent that get scary fast. Not much has to go wrong for that to end badly.
So please come and enjoy the challenge and beautiful Vermont but take care to get home safely!
Charles M. Coldwell, W1CMC
Belmont, Massachusetts, New England
"Turn on, log in, tune out"