Short answer: Revise your core subjects from standard textbooks, then practise with previous year GATE papers. Choose according to your branch.
Standard textbooks
The books you used in college are often enough for concepts. Revise them with the GATE syllabus in front of you.
Problem practice
Add one problem book per subject for numerical practice, and focus on weak topics.
Previous year papers
Solve topic-wise and then full papers. They show how questions are asked and how to manage time.
Related: sell GATE books.
Frequently asked questions
Do I need to buy new GATE books?
Not necessarily. Many students reuse their college textbooks and add a few practice sources.
Can I buy GATE books second hand?
Yes for the right edition and condition.