vast nature. However, it is one of the easiest sections to prepare for. The trick with this section is just to scratch the surface. No need to go deep into any of the subsections. During my preparations, I mostly concentrated on understanding the definitions of all the terms and only the most important results. Again, as in all of the GATE prep, the operative word is "understanding". Old GATE papers were more than enough for practice. Now, I will try to walk you through each section.
I personally did not use any material for preparation of this section. Just solved questions from old GATE papers and that was it. One only needs to understand what all the operators mean and just around 10-15 basic formulae. One can safely rely on old question papers for preparation of this section.
For the basics any good 12th level book will do. Thoroughly understand the concepts of event, sample space, conditional probabilities. Bayes theorem is especially important. I did not cover mean, median, mode and std deviation as I was already comfortable with them. I think 12th level statistics should cover it in adequate depth. For understanding random variables and other (somewhat) advanced topics (eg. distributions), "A First Course in Probability" by Ross should be enough. Just be very selective, no need to go beyond the basics.
Set Theory & Algebra:
I studied this part from Schaum's Discrete Mathematics. One just needs to know all (around 20-30 odd) definitions and how to read the lattice diagrams.
I covered all of graph theory from wikipedia and it was more than enough. Just look up the relevant articles in wikipedia and try to understand important definitions and theorems. Again, don't go in too deep.
For the basics, stick to your 12th textbook. Do Gaussian-elimination from Gilbert Strang. For eigen values and vectors, you just need to know what they are and how to solve for them. Anywhere you can get this information is fine. I think Gilbert Strang does a good job in this area too.
LU decomposition is part of Gaussian-elimination, and that you should have covered in linear algebra. For the rest its wikipedia all the way.
I did not cover this topic. Knew enough of it to answer asked questions. Mostly they stick to 12th level stuff. Anyway, very few questions are asked from calculus. You may leave this subsection (at your own risk)
Hope this helps.
Do pitch in with your comments and queries.