Not necessarily to appear for GATE Coaching. Woth studying from right syllabus and knowing exam pattern and solving maximum mocks you can surely clear.you must study from right set of books and analyse all mistakes committed in mocks. Revise those sections and practice more. Solve out all doubts as well. 

List of all bestbooks for GATE below. 

https://engineering.careers360.com/articles/what-are-best-books-for-gate-preparation