I don't understand TOC, or subject X. Should I study it?
You can drop a subject if you hate it. Concentrate and focus more on subjects that you are interested with.
I'm presenting here links to valuable online resources for GATE in the form of PDFs, PPTs, applets and lecture notes. The top 3 links are actually on my servers and they contain standard books in PDF format, and previous year GATE papers as well as some GATEFORUM's practice papers.
1. http://indifun.in/gate/resources
Collection of text books
2. http://johnpatrick.vndv.com/gate
Collection of more text books as well as several PPTs
3. http://johnpatrick.vndv.com/gate/papers
All previous year papers
4. http://hpc.serc.iisc.ernet.in/~govind/hpc/
Awesome collection of lecture notes from India's best T-School i.e.
Indian Institute of Science Bangalore(IISc, Bangalore)
5. http://www.itl.nist.gov/div897/sqg/dads/
Dictionary of data structures
6. http://www-math.cudenver.edu/~wcherowi/courses/m4408/gtln.html
Graph theory material
7. http://www.csanimated.com/animation.php?t=B-tree
An ultimate guide to B-Tree data structures
8. http://webster.cs.ucr.edu/AoA/Windows/HTML/MemoryArchitecturea2.html
Memory architecture
9. http://www.d.umn.edu/~gshute/arch/cache.html
Jargon buster for cache memories
10. http://www.cs.iastate.edu/~prabhu/Tutorial/title.html
A very good tutorial on Computer Architecture
11. http://www.cs.umd.edu/class/sum2003/cmsc311/Notes/
Basic concepts in computer organization
12. http://www.mec.ac.in/resources/notes/index.html
Online DS and Automata material from MEC
13. http://www.cs.odu.edu/~toida/nerzic/content/schedule/schedule.html
Discrete mathematics material
14. http://www.ugrad.cs.ubc.ca/~cs421/notes/
Automata theory excellent stuff
15. http://people.csail.mit.edu/rinard//osnotes/
Operating Systems material
16. http://www.nos.org/htm/course.htm
Some very basic stuff for beginners
17. http://www.gateguru.org/automata.html
Automata theory previous years GATE papers solved
18. http://cpualgorithm.blogspot.com/
CPU Scheduling example code in C
19. http://www.po28.dial.pipex.com/maths/formulae.htm
Some handy mathematical formulae
20. These are links to useful material from NPTEL(NATIONAL PROGRAM FOR
TECHNOLOGY ENHANCED LEARNING) program run by IITs, which puts all
their lecture notes on the web.
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-%20Guwahati/comp_org_arc/web/index.htm
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/compiler-desing/ui/TOC.htm
http://www.cdeep.iitb.ac.in/nptel/Computer%20Science/Design%20and%20Analysis%20of%20Algorithms/TOC.htm
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-%20Guwahati/data_str_algo/frameset.htm
http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-%20Guwahati/afl/index.htm