GATE is one of the toughest exam to crack.

If you want to crack GATE in 40 days you can start your preparation with the topics which carries more weightage in the exam.

Try to clear your concepts.

Solve previous year questions papers to understand the exam pattern and level of the questions.

Revision plays a good role to increase your score. You can make your important notes which will help you at the time of revision.

To prepare for aptitude you can refer to Quantitative Aptitude For Competitive Examinations- by R.S. Aggarwal and A Modern Approach to Verbal & Non-Verbal Reasoning- by R.S. Aggarwal

Good luck!!