GATE

Hi,
You should go for GATE, if you want to be in same field because CAT is common admission test which is conducted to get admission for MBA program, which is a management course.

You are not eligible to appear for GATE exam as minimum eligibility is 4 years degree course after class 12th to seek admission in M.Tech CS.

You can check the below mentioned link:
http://www.gatesyllabus.in/about-gate/gate-exam-pattern

Most of the PSU companies offer jobs to civil engineers on the basis of GATE marks.

Made Easy is the most renowned and they have maximum selection every year.

Hello Zeeshan,
There is a huge competition in GATE and its completely uncertain to get a seat until/unless you are so talented.
But when it comes to GRE, there are thousands of options available. If not admitted to one university, you can get in another but same is not the case with GATE.
Better choose GRE and try for scholarship.
Also, if you think you can crack GATE then go for it.

Eligibility for GATE examination is a B.Tech/BE/AMIE or M.Sc. After 12th you can appear for JEE.

Eligibility is a Bachelors degree in Engineering/Technology (4 years after 10+2 or 3 years after B.Sc./Diploma in Engineering/ Technology) and those who are in the final year of such programs.

It is not mandatory to take admission, if your scores are excellent in GATE, then you can get PSU sector's jobs easily.