As you are aware, the GST is administered both by the State Governments & Central Governments. As per the decision taken in the GST Council, the Taxpayers will be allocated either to State Government (or) Central Government.
The Tamilnadu Government vide its Order No 1/2018 dtd 28/02/2018 has published the complete details of the allocation of GST taxpayers allocated to Centre and State Government (Tamilnadu)
The notification link is provided below. You can download the respective documents and search for your GSTN to know where your case has been allocated.
Click here for list of Taxpayers allocated to State Government (Tamilnadu) – (This document is 30 MB size maynot display correctly in mobiles, suggest to view in Desktop)
Source :- https://ctd.tn.gov.in