gst penalty or interest

late fee and interest imposed when not filed on due date.
Late fee for Nil Return - Rs. 20/day (CGST Rs 10 & SGST Rs 10)
Other than Nil return - Rs. 50/day (CGST Rs 25 & SGST Rs 25).
Maximum late fee Rs 10000 /month.

Also pay 18% per Annum interest for late payment for all outstanding payments. 

Late fee is auto calculated by gst portal while submitting returns.

Agree with Mr Vinod Ji
Upto September 17 late fee for 3b has been waive of by Notification 28 & 50 of CT . hence till sept 17 no late fee is attract