I drive on gravel if it is on a route I want to take, so say 50kms of gravel would be ok if the end was worth it. For many years going to bike rally meant gravel, lots of it and not that great. As Pete says, keep loose, be sensible. The ST handles gravel better than the rider usually. In the really loose stuff, such as sandy roads, I was taught in a stay upright course to gently wiggly the bars to prevent the sand building up on the front wheel so you could keep on going. Worked the few times I went on really sandy roads, and that was 2 up with camping gear on for the weekend.
Always look at where you want to go, not stare at the front wheel and stiffen up, as where you are looking is where you will end up.