GPD is an acronym for gallons per day, while MGD is an acronym for millions of gallons per day. Both are used for rates of flow of a liquid, but the former is appropriate for small flows (for example, how much you water your lawn) and the latter for much larger flows (for example, the total amount of water used on all lawns in New Jersey).
Write down the number of gallons per day (GPD) that you want to convert. For example, 1020 GPD.
Divide by 1,000,000.
Observe the result, which is the equivalent number of millions of gallons per day. In the example, 1020/1,000,000 = 0.00102 MGD.