is it mandatory to file gst return or not?

AS PER MY OPINION ,

NO AS I ALSO FILED APPLICATION FOR CANCELLATION ON 05-01-2018

IT IS STILL PENDING 

HOWEVER , IT IS NO NEEDED TO FILE RETURNS

Cancellation of registration is not your right unless the law deems it so on expiry of prescribed period (if any). The proper officer has to examine your liabilities of taxes etc. as per law before cancellation is allowed. Therefore, it would be appropriate to file the returns till cancellation is formally done.