Hello Aspirant,
Gat-b and jnuceeb are the same examinations. Earlier the examination were conducted by jnu but now it's conducted department of biotechnology hence it's renamed to gat-b.
Coming to the eligibility criteria, students who are Undergraduate can apply, thus its a competitive exam for applying for the Postgraduate. If you're an Msc student and want to do Mtech, then also you can give the Gat-b examination. Gat-b examination paper pattern consists of two parts: Part A, containing 60 mcq question containing 1marks compulsorily to attend and Part B contains 100 mcq questions but you can attend any 60questions.
Yesss students from bsc livescience can give these examination for further studies.
You can go to the official site to apply for the examination:
rcb.res.in