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No. GATE is not mandatory, but if you are thinking to do masters in India, then GATE is the best. Even if you spend a year in preparation, no problem, go for GATE.
Without GATE-
1.Career aspect: First thing you can't get into well reputed institutes like IITs, NITs, and some Govt colleges. When you done with you are M.Tech, the college from where you have done your masters is very important. It matters a lot in initial stages of your career. Most of the private colleges in India for name sake they are providing M.Tech degrees, but they don't have required lab facilities and well qualified faculty too, so what is the use obtaining degree from such universities?
2. Financial aspect:You have take care all financial needs by yourself. If you have cleared GATE, you will be getting stipend of 12400 pm,that too in Govt institutes you will have very less fees. Do you know? IIT Kharagpur tuition fee is 5k per semester, including everything ie food+Hostel+internet+medical etc. it comes out 33k per semester. You wont get such low fees in anywhere in India.
Actually any GATE qualified , if he takes admission through GATE in any AICTE approved colleges should stipend of 12400 pm, but it never happened in reality. You know its all government, no need to explain our system.
My advice, if want to do M.Tech, do it in well reputed colleges through GATE. If you are just thinking to get a M.Tech degree, then do it some private colleges.