The question, as stated, cannot be answered sensibly. A gallon is a measure of volume, with dimensions [L3]. A milligram is a measure of mass, with dimensions [M]. The two measure different things and basic dimensional analysis teaches that you cannot convert between measures with different dimensions such as these without additional information.
As a simple mental exercise consider a gallon of air and a gallon of water. They will have very different masses.
1 mg equals how many grams?
1000 mcg equals 1 mg.
1 US gallon = 3785.41178 cc
2 qts 1 gallon = 4 quarts 1 quart = 0.25 gallon
1 g = 1,000 mg 5 g = 5,000 mg
1 mg equals how many grams?
1 gallon equals 1 gallon.
1 US gallon = 3.785L
1000 mcg equals 1 mg.
1000 mcg equals 1 mg.
1000 mg equals 1 gm
1 gallon equals how many quarters
1 gallon!
1 gallon equals 8 pints.
1 US gallon equals 3,785 cc, and 1 imperial gallon equals 4,546 cc.
1000 mg, for every 1000 mg equals 1 g.
Milligrams, a unit of mass, and gallons, a unit of volume, are not actually directly equivalent, however it is common to use the volume and mass of water to "equate" the two. Using that method, where 1 (U.S.) Gallon of water weighs 128 ounces, and 1 ounce weighs 28.35 grams (approx.) or 28,350 mg 1 Gallon "equals" 128 * 28,350 mg, or 3,628,739 mg