i am getting 42 marks in gate 17 cse set 2. is there chance of getting nit? and if so should i go for it or prepare again for gate 18?

The score is not that good to get most of the top NITs but if you want you can check where all you have admission chances by using below college predictor -
http://www.engineering.careers360.com/gate-college-predictor

And I would also suggest that if in worst case you dont get any chance to get admission in NITs/IITs then you can try for good Govt state colleges along with that try for BSNL JTO also, below link will provide detailed information about BSNL recruitment-

http://www.engineering.careers360.com/news/bsnl-announces-recruitment-of-jto-through-gate-2017-214850

Or you can also go for a job alongwith that keep preparing for Gate 2018.

So decide wisely.

Good Luck!!!