I am getting two dates - 200 BC if you read Brassey and 198 per Dupuy or Kohn.
Or 201 BC in the Perseus translation of Polybius (http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0234%3Abook%3D16%3Achapter%3D18).
General consensus however seems to be 200 BC, with Gaza subsequently taken in 198 BC after a siege. I would go with 200 BC.