Ans:
In this case, the site location is considered to be the place of supply. In your case, the place of supply is Ahmedabad, and Ahmedabad GST is applicable. Both CGST and SGST will apply. Ensure you are registered under GST in Ahmedabad or Gujarat. You can also register as a casual taxpayer for this order under Gujarat GST, wherein you get 90 days to complete your work. Just in case it is incomplete, you can request an extension showing due cause for another 90 days.