Which is better for post graduation in civil engineering? NICMAR or M.Tech through GATE?

If you really want to pursue a career in the Technical field then it will be suggested to go with M.Tech as you might get ended up in some renowned structural engineer. You can also pursue any other subject as well if you are interested. 

This will open an opportunity in doing a lectureship in college. By doing so, you can further pursue Ph.D.

It would be highly suggested to go with GATE only if you are getting into top IITs and NITs as it will make an impact on future references.

Apart from this, if your interest area is in Civil engineering but in a different domain, then go for NICMAR as it will open the option for accounting, marketing, business development, contracts, planning, valuation, execution, etc. By doing so, you might as well end up as a successful corporate or entrepreneur.