I has done it recently in a fit of madness. The G/F's DT 230 has been off the road for a couple of months for an engine rebuild. Back on the road now & as part of the running in time I thought it was a good idea to do GP 8's on gravel. Erm, skills wise it's like stropping your razor, brake, throttle & grip are tenuous as speed builds up, it goes wrong very quickly. Makes you get on the throttle very early & stops you from braking to turn, squeeze & go, no holding the brakes as you lean. Same with the throttle, no messing about, the bike starts to slide you have to gas it, sideways. Which wrecks your lines. Bloody hilarious & very sweaty, though.