Your best bet for close gravel is reformatory branch/minuteman loop. I did that on a weekly basis for years, while living in Arlington. There’s also shorter gravel (and good MTB routes) in the fells. Check out JRA cycles.
I now live in VT, which has closer to the terrain your used to, with general rides being over 1000’ per 10 miles, so expect to find your way north from time to time.
If you have any interest, think about getting involved in cyclocross, the boston area is great for that, and theres even two option for wednesday training races (morning or night)
Honestly, for living in the city, I do beleive he mountainbiking can be quite good. Youcan ride to the Fells, or the western greenway quite easy. Plenty of good driving options inside of 40min.
I would imagine most your rides on the roads will reach towards lexington, concord, carlisle, lincoln, sudbury, etc. Generally some great rides out to Harvard (the town) as well, heading through the forementioned towns and acton.
I never liked riding south, except for group rides, north is tough, theres sort of a gap in “safe” roads. Using the minuteman bike path (or mass ave) to get west, quickly was always my choice.
MA drivers kind of suck, but most areas around populated cities do, so im not sure its worse.
This is the reformatory/minuteman gravel I was talking about, just use the paved minuetman trail to get there from arlington: Concord Gravel | 17.7 mi Cycling Route on Strava
something like this was a ride that I had many variations on: RD_Arl-Con_39 | 38.9 mi Cycling Route on Strava