"gm" is a standard abbreviation for the metric weight "gram" (The other one is "g"). Anyone who is confused or suggests otherwise needs to brush up on their units of measure.
The metric system is smart because:
1 gram of water = 1 milliliter of water
That only works for water, but water has a standard density to make such calculations easier. It allows us to convert units of mass (grams) to units of volume (mL). That means you have:
120 mL of water
Now you want to convert units of volume to units of volume. Use any conversion tool you like to finish the question:
http://www.google.com/search?q=120+ml+in+cups
It is almost exactly 1/2 cup.