Hello,

GATE is Graduate Aptitude Test in Engineering. GATE is conducted for admission of M.Tech as well as for jobs. There are many diverse papers available in GATE including that of biology. So I think first you have to find out in which field you want to build your career. Then after that look whether you can cope up with the B.Tech in that stream, if not try another stream. Once you fix a stream go on with that. For engineering there are many papers for GATE. So when you are applying for GATE, apply for those paper(s) for which you are confident to crack. Then your core subject of B.Tech. will be at the first preference.

it matters if you are looking to get into PSUs,but doesnt matter if you want to do Mtech.

many PSUs have vacancies in specified branches and you need to have a degree in that branch only. 

The most favoured branch if you are looking for gate would be mechanical, civil and electrical. These have greater number of seats comparatively.