Source : Webster's Revised Unabridged Dictionary (1913)

Gag law \Gag law\ (Parliamentary Law)
   A law or ruling prohibiting proper or free debate, as in
   closure. [Colloq. or Cant]

Source : WordNet®

gag law
     n 1: a rule for limiting or ending debate in a deliberative body
          [syn: {closure}, {cloture}, {gag rule}]
     2: any law that limits freedom of the press