Even i didn't knew my "Sector / Circle / Ward /Charge / Unit" for GST registration & as we are registering for sales/service tax for the fist time, i also didn't had any Acknowledgement Certificate from taxes paid previously.
so; i took an old purchase-bill of a product which i purchased from a near-by shop & searched it's TIN number at my state's sales-tax website - https://cg.nic.in/AnnualReturn/OtherServices/tinSearch.jsp
in the result for TIN wise Dealer Search i got the "CIRCLE" name for the TIN and i filled the same in my GST application (as our offices are in the sme area).
THIS IS JUST A SUGGESTION.
KINDLY SHARE IF ANY-ONE/YOU HAVE A BETTER IDEA.