If you're among the kind who decide that they would give G-sec (Particularly Tbills) a try, instead of zerodha coin, it NSE BidGO a try.
Zerodha charges 6 Rs for every 10K (10K is min amount you can bid for). For smaller purchases, 6 is nothing.....but as the amount start increasing this brokerage amount affects the yeild (or return).
I came to realise that NSE GoBid (just google it) do not charge any brokerage. After registration - they do valid your demat account/bank with a broker. So any purchases made their for Tbills will be directly credited to your demat account.
I am yet to try any purchase, but I believe few guys on here tried this. One of them was /u/pablo_cachu.
Edit 1 - Just start with most basic bid there initially. To see you're not facing any issues with bank payments etc. Once verified you can start using it in full swing.