The actual units is mostly irrelevant in the game unless your trying to calculate a position or velocity based on free fall. 800 = 100% of "normal" gravity. So, if you set it to 400, it's 50% of normal.
Gravity is a unit of acceleration which is distance divided by time square. On earth, gravity is approximately 9.81 m/s^2. If we assume that 800 is earth gravity (which assumes the original developers also made this assumption) then 800 units is equal to 9.81 m/s^2.