I've raced the BF1 against the RB4 - the BF1 is much slower in the dirt sections. But the RB4 gets owned in the tarmac sections, so overall the time is lower for the BF1.
Also it helps if you take a massive wide line into T1, this allows you pick up much more speed before hitting the dirt. Once on the dirt, you've only got enough traction to keep your speed up, so whatever speed you hit the dirt at you tend to carry though (it's an exaggeration but you get the idea). So driving this horribly wrong line helps a lot, in the BF1s case.
I think Mika Häkkinen did drove F1 car on snow for some commercial clip of Mercedes, so gravel, why not
I did some rallying today, Blackwood reversed rally track, there I did saw BF1 too doing 1:05, I'm not very good and my best lap with RB4 was low 1:11 I think so no way to compete with F1 :P