With GATE you can just focus on studying the topic and not worry about the job related work. For people who are in TCS, Infosys (or bigger companies) even if you learn machine learning, its not that easy to switch to that department which does only data science.
From IITs, even if Mtech, you can get hired at a higher package than your previous engg. college (assuming non-IITs) and maybe directly for data science department.
That being said, these are not the rules. You may be able to find a company/department and apply for a data science position and get selected if you just learn machine learning and clear their interviews. Some companies care about your pedigree, some don't.
Some people just want to study more before grinding in the industry.
Your mileage may vary.