GATE

No, it is not necessary, but GATE is an important exam to improve our skill, and I think GATE is compulsory for graduation. And it helps you in achieve good rank in PSU under direct basis of GATE and you are eligible to get job.

You must join a coaching institute for GATE 2018 batch. You must take a crash course. You can also join the test series for GATE exam in 2018 batch. Better to join Made Easy coaching.

Arihant GATE tutor or G.K. Publication.

Yes, you can opt for avionics as the specialisation.

You well know that Engineer at a reputed PSU have more powers in comparison with the Bank PO. Not only power but the money too. Your life would become still and monotonous if you go for Bank PO while in the other case, you never know what you explore in the process.

After B.Tech those students who failed to get placement then they go for GATE exam or those who wants to go for further study.
Listen by giving GATE exam you are eligible for higher education and one more qualification added to your bio. that you cracked the gate exam so when you go for job in company then they give you more preference then a B.Tech passed student.
Regards

Tarun, you can take admission without GATE. The eligibility for M.Tech is
Pass with 60% aggregate marks in B.E. or B.Tech. or M.Sc./ MCA or equivalent, subject to qualifying LPUNEST-2017 or GATE.
OR
In case marks scored in graduation are 70% or above, then LPUNEST-2017 or GATE will be exempted and candidate may apply for admission.