You’ll want to get out of Cambridge as quickly as possible. It may be Bike Friendly, but the drivers here are lunatics and most of the roads suck. Urban riding at it’s finest.
Fortunately, Minuteman Bike Path goes out to Bedford and then plenty of loops from there, so you can get out of town quickly.
No gravel that I’m aware of without driving, but plenty up in NH. With the exception maybe being the Middlesex Fells (near me) in Medford, but I prefer that on a MTB.