Keep the gram value the same, since you are using grams as the numerator (the top number in a fraction) for both measurements.
Multiply grams per kilogram by 104 to convert to grams per cubic meter. This number is the molality constant (molality is a standardized measure of the composition of one substance in another substance). For example, if you have a substance whose concentration is 10 grams per kilogram, multiply 10 grams by the constant 104, which yields 1,040 grams per cubic meter.
Consult a chart of density values (see Resources) if you know what substance you are working with. For example, the density of water is .92 grams per cubic centimeter.
Divide the kilogram value by the density to convert it to cubic centimeters (which is the standard measurement for density), and then divide by 100 to convert to cubic meters. Again, the gram value stays the same. Imagine, for example, that you have 100 grams of gold per kilogram. The density of gold, according to the chart, is 19.3 grams per cubic centimeter. Divide 100 by 19.3 to get 5.18 grams per cubic centimeter. To convert to cubic meters, simply divide 5.18 by 100, which is .0518 grams per cubic meter.