Sir,
As per Section 142 (8) (a) of CGST Act, 2017 " where in pursuance of an assessment or adjudication proceedings instituted, whether before, on or after the appointed day, under the existing law, any amount of tax, interest, fine or penalty becomes recoverable from the person, the same shall, unless recovered under the existing law, be recovered as an arrear of tax under this Act and the amount so recovered shall not be admissible as input tax credit under this Act.
Therefore service tax if not paid under the existing law is recoverable under CGST Act as arrear of tax under CGST Act and the amount so recovered shall not be admissible as input ta credit under CGST Act.