How do you stop a car if sliding on loose gravel down hill?

Locking-up your brakes is almost never a good idea except in this situation.

If you find yourself sliding and you're on loose gravel or dirt or even snow, Lock-Up your brakes! This will create a build-up of stuff under your tires and stop you faster.

But you're at the mercy of the law of physics. Whichever way you were heading before mashing on your brakes is the same direction you're going to continue until you stop or let off of the brakes allowing your tires to get traction again.