Almost any gouge can be repaired using a paste made from epoxy and a powdered form of the original material.

For a marble slab, if it's regular run-of-the-mill white marble (or if a 100% match isn't an issue) mix up a repair paste from pulverized marble and epoxy. Just get hold of a few marble chips (sold for landscaping or aquarium use) and beat hell out of 'em with a hammer. Then mix the epoxy according to directions and add the marble powder. Try a few different ratios of powder to epoxy until you find what seems best. I'd start with 70% powder/30% epoxy, but that's just a guess. Be sure to use a good grade of slow-drying non-yellowing epoxy.

If it's a colored marble, try to find rocks that are a close color match and use them instead of plain marble chips.