GATE

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.

Assistant Engineer Civil (Level I) is the trainee level class in the professional civil engineering series. Incumbents perform field and office assignments of a less complex nature in connection with the planning, design, construction and maintenance of a wide variety of civil engineering projects. Incumbents are expected to demonstrate increased proficiency during the training period, and, upon completion thereof, promote to the Assistant Engineer Civil (Level II). Incumbents are gradually given more difficult and responsible assignments and may assist experienced engineers in charge of more difficult civil engineering projects. Assistant Engineer Civil (Level II) is the journey level class in the professional civil engineering series. Under general supervision, incumbents are responsible for routine office or field projects of average difficulty and are expected to direct them to completion. Incumbents assigned to Level II generally have, in addition to meeting the minimum qualifications for the class, at least one year of paraprofessional engineering experience equivalent to one of the following classes in Sacramento County service: an Assistant Engineer Civil (Level I), Senior Engineering Technician or higher. A Master's Degree in Civil Engineering may be substituted for the experience. An Assistant Engineer Civil (Level II) may serve as a resident engineer on smaller, less complex construction projects. Civil engineering work of this type is generally supervised and reviewed by positions in the next higher class, Associate Civil Engineer, which is assigned the function of squad or project leader. Assistant Engineers Civil (Level II) have no supervisory responsibility, but may provide technical direction to subordinate engineering and paraprofessional engineering personnel engaged in design and construction of such work.