Many would say 3 to 4 month serious preparation is enough but I would suggest you as per present competition you must start your preparation as soon as possible targeting GATE Examination
1. First of all go through latest syllabus of GATE completely. Read every single topic of each subject carefully mentioned in the
“It is important to know what to study but
it is more important to know what not to study”
Because you have to choose subjects to study as per GATE syllabus you do not need to study all subjects which you have studied in your B.E. / B. Tech. course.
2. Solve GATE previous years papers.
3. Always read standard books.
Why need to read standard books ?
Because standard books written by highly experienced Professors of reputed institutes / universities. Data available in those books are very well examined on theoretical as well as practical measurements. These books always comes with complete analysis and statistics, which you can find on the start and last of the books.
GATE papers are designed by IISc, IIT highly experienced professor those who have written many international and national level books and articles. So always try to use standard books try to avoid mere knowledge books.