Hello Bablu.

GPA in BTech is either for higher studies like M.Tech or for join a company.

For placements, GPA is required for a eligiblity to appear for the interview. So if your GPA is less then you won't be allowed to appear for the interview no matter how good your GATE Score is. But if you are eligible for the interview, i.e. if your GPA is more than the minimum required criteria for appearing for the interview, then your GPA will not much mattered by the interviewers. The matter of GPA after being eligible for the interview depends on the company and interviewer. 

I hope it helps you.