Density and specific gravity are related but not the same, Specific gravity is defined as the ratio of the density of a substance to the density of a reference substance. In this case, the reference substance is water, and it is commonly assumed to be water at 4°C (39.2°F) and is generally given as 1.000 g/cm3 (or 1.000 g/ml).
So, the specific gravity of grain alcohol = Density of grain alcohol / Density of water at 4°C = 0.789 g/mL / 1.00 g/mL = 0.789.
This value is dimensionless and it is used to compare the density of different substances with water.