Is dropping 2 year for gate CS valid as I don't want to write these employbility test like elitmus, amcat etc who offers very less package.

There shouldn't be any problem as far as I can see. GATE does not have any restriction. As for M.Tech admissions, the GATE scores will matter. PSU recruitment through GATE also is no problem. As you pointed out, it is late to apply for GATE 2018. My only advice would be to seriously study well as 2 years is a long time and CS is a very popular branch with a lot of competition.