Can we avail closing input servicetax with gst tax amount??

No

May be allowed only after filling GSTRN 3

first you have to file TRAN1 from before 30th septemeber 2017,  after the approval of government for the TRAN 1 form wherein you will declare the input tax credit  balance as per your 30.06.2017 return and other relevant details you can utilise the openiong alance to pay tax.  as of now the tax for july has to be paid before 20th august as from 3B has to be filed by 20th august.  hence you may not be able to use the  credit availble as on 30/06/2017 for payment of tax under GST regime for July 2017.