1 gram per liter = 0.00834540445 pounds per US gallon, so multiply the number of grams per liter by 0.00834540445 to get pounds per US gallon.
Multiply grams per liter by 0.1 to get grams per deciliter.
1 microgram = 1 millionth of a gram or .000001 - so, just move the decimal point over to the appropriate position. As long as you don't change the value of the number, you can do what you want to it.
Multiply grams per liter by 0.001 to get grams per milliliter.
Multiply by 1000. A liter is 1000 times bigger than a milliliter.