In my opinion, you should be at least very strong in basic common important subjects for GATE.

Here I'll provide the subject with the books Author name.

1. Thermodynamics(P.K. Nag)
2. Machine Elemnts (V.B. Bhandari) 
3. Fluid Mechanics 
4.Material Science(O.P. Khanna)
5. Automobile (Kripal Singh)
6. Strength of Material (R.K. Rajput)
7. Dynamics of Machine
8.IC Engine (V. Ganeshan)
9. Engineering Mathematics\
10. Quatitative Apptitude(R.S. Aggrawal)

You can get the Syllabi of the GATE from Page on iisc.ernet.in

Let me tell you these book can help you to build the basic concept and understanding, and also to recall what we studied in the previous semesters.

I forgot the Author of some books, apologies for the same.

Next Step, 
Once you build the concept and basic understanding, just purchase a past 15 Year Papers of GATE with solutions, and solve it thrice.

This will help you to understand the pattern of the question asked in GATE as well as some question might REPEAT itself.
In my GATE 10 questions were repeated, so when you solve thrice, at the exam time you just recall the answer and move ahead. This saves TIME.

hi !! 

please refer to the site given below to know about some really good books 

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