Join only top 3 nit's otherwise you will end up the same jobs that you don't want to do. 2 Year gap is kind of ok if you end up in a good college. Companies in good college does not see gap while they recruit on campus. Only boundation is the cgpa you have in MTech.
But in case you fail you have to explain the gap in open market, many company will see it as a negative point as well. As stated earlier you cannot guess what will happen on D Day. It's a kind of risk. It's your life you have to make a decision. Only remember life will not end if you fail again. Even if you fail 4 times life is always beautiful. Some points from above comments also make sense.
1. Do you REALLY want to go to IIT/IISc ?
2. Can you afford One year drop (Financially, Economically etc)
3. Do you believe in yourself that you can do it ?
4. If you drop, Will you give your best and will put efforts that are required to crack GATE ?
If the answer to all these Questions is Yes, then Go ahead. Glory is awaiting you.