Contract receipt including GST-how to calculate taxable supply - Goods and Services Tax - GST

Sir, our client is providing woarks contract services to state govt departments.The work contract terms says that the tender amount will be including GST the GST will be borne by the contractor i.e. the department is not giving us separate amount for GST. While filing 3B we are taking taxable supply net off GST ie. amount as reported in GSTR-7 by the deducter divided by 1.18 then on the net amount the 18% GST is worked out. Due to this amount as per GSTR-7 amount as per 3B is mismatched due to that GST department issued notice to explain the difference.Plz guide whether we are filing GSTR 3B in correct way or we should pay GST on the gross amount. any notification/circular in the matter.