i m confused in gay lussac law....how 100 ml of nitrogen combined with 300 ml of hydrogen to give 200 ml of ammonia//plz explain

N₂ + 3H₂ = 2NH₃

According to gay lussac law V is directly proportional to number of molecules.

1 mole of nitrogen + 3 mole of hydrogen = 2 mole of ammonia

100ml + 300 ml = 200ml