Start with the kitty litter or sawdust. Leave it for a day, sweep. I'd do that twice myself, since kitty litter is cheap. Engine degreaser is a good idea for the next application. I sure would not do gasoline, Gasoline vapors are very flammable. You want to clean the garage, not blow it up. Keep the door open while your using degreaser so you don't get flammable vapors building up. After degreasing, I'd do kitty litter immediately after, but you can just leave it there for a couple hours this time rather than overnite.. Also, a power washer could be very helpful. Use the kitty litter in between each application of solvent. Water counts as a solvent, since it is a solvent.