7 speed? Wow, might be a little older? I have an early '90s scott mountain bike that does great on gravel. It was a light weight mountain bike, so probably weighs 24lbs. Its got fairly modern gravel geometry, probably on the aggressive side (short head tube).
My biggest problem is finding decent tires and wheels. Most of the good stuff has migrated to 27.5 or 29" leaving behind cheap commuter tires or heavy duty touring tires. Nothing much that I would enjoy on a long gravel race. I stocked up a bit on 26" closeouts, but not sure if I'll have any tire options after that.