Every person registered under GST Act has to periodically furnish the details of sales, purchases and tax paid and collected theron by filing return with GST Authorities. Before filing any return payment of tax due is pre requisite otherwise such return will be invalid.
The limit of Rs. 20 Lacs is regarding GST registration. Once you register under GST you have to compulsorily file the return.
Even if there is no sale and purchase during the period, the registered person has to file NIL return within due date.