The problem is probably not that there aren't routes, but rather how to find them. Gravel isn't about pure asphalt or difficult mountain trails - it's about the mix, so you need to change your search approach. I spent a long time looking for decent rides until I realized I needed to look at forest roads built for forestry services and wildlife protection. They're exactly what you're looking for with gravel: gravel chips, dirt, sometimes asphalt sections, all in forest areas rather than on main highways.
Around Moscow there are tons of such places. I can recommend areas like Sokolniki, and heading southeast toward Egorievsk, or areas around Taldom and Dmitrov to the north. Lots of interconnecting forest roads, few people, and the surface is exactly what you need for gravel. Two to three hours max from Moscow by car. I often go there on weekends myself because riding the same route five times gets boring.
A tip - get yourself 2gis or even Google Maps and look at the satellite view. You can see all those forest roads that often aren't marked as routes on regular maps. Then you just check them out in person. Yeah, the first few trips might be a bit haphazard, but that's how you'll find your own spots that aren't all over the internet.