Lay the fish down on the grill with the tail facing farthest away from the heat. It will cook much faster than the head end, even with the slashes you made. It is very important that you do not grill large fish over very high heat! They will burn to a crisp on the outside before the center is cooked through. Steady, medium heat is what you want.
How long? About 10 minutes per side on a 20-inch fish such as this striped bass. The general rule is ten minutes per inch of thickness.
Only flip your fish once. It is a difficult operation that requires you to carefully lift the fish with two spatulas (or one long one), and GENTLY flip it over. If you have done everything correctly, you will get little or no skin sticking to the grill.
Does sticking happen? You bet. Don't fret, the fish will still be tasty!