If a candidate has scored above the cutoff in iit, do he has to face another screening from iit before admission
If you have marks above the cut off for the JEE ADVANCED, then you can give the advance for getting selected in top IIT'S, but if by chance you didn't make into IIT'S, Then by God grace you have already cleared JEE MAINS, and you will get any NIT'S, upon your AIR rank.
If you are talking about mtech through gate, then in some courses you have to give tests and interview while for some there is direct admission.