Actually locking up your brakes works on gravel because it builds up a dam of gravel in front of your tires.
On our gravel roads best braking is when the wheels lock and dig through the loose stuff to the solid layer underneath. ABS stops this happening by preventing lock up, so you're skating on the pebbles.
S550 drivers report over at TrackMustangsOnline that they get something called Ice Mode when they use sticky tires and high friction brake pads on track. The ABS activates and reduces their braking ability. The S197 cars were really bad about it.
