Gravelmap is a pretty handy resource for stitching together gravel roads in CT: https://gravelmap.com/#8.69/41.5885/-73.0269

Like a bunch of other folks said, the Airline Trail is fantastic. It intersects with the Hop River trail in Willimantic as well. In that same region, there's also some fantastically hilly dirt roads that run through the Meshomasic State Forest. Some pretty sick 10% grade roller coaster stuff in there, I absolutely love it.

It's possible to stitch Airline, Hop River, and The Mesh together to create a pretty huge and unforgettable gravel loop. You can also check out the maps on Trailforks, you'll notice that there's a whole bunch of singletrack MTB trails that intersect the rail trails. So there's plenty of opportunity to get rowdy and underbike it on a 650B wheel set too.

You should also plan on checking out next year's Nutmeg Nor'easter event. The photos speak for themselves: https://theradavist.com/6th-annual-nutmeg-noreaster-an-alternative-new-england-experience/