You have two options to go with,

The first one is just to buy either ace academy or made easy study material and buy reference books of those subjects only in which you are not comfortable.

The second option is to buy reference books of all subjects and previous year questions book from either ace or made easy.

So I am listing some of the good reference books which will be helpful for your gate preparation.

Circuit Analysis:- Fundamentals of Electric circuits by Charles K. Alexander, Matthew N. O. Sadiku. Or Electric Circuits from Schaum series by Mahmood Nahvi, Joseph Edminister.

Transducers:- A course in Mechanical Measurements and Instrumentation & Control by A.K Sawhney.

Digital Electronics:- Fundamentals of digital circuits by A. Anand Kumar and Digital logic & Computer design by M. Morris Mano.

8085 microprocessor:- Microprocessor architecture programming and applications by Ramesh Gaonkar.

Analog Electronics:- Electronic Devices And circuits theory by Robert L. Boylestad, Louis Nashelsky AND Op-Amps and Linear Integrated circuits by Ramakant A. Gayakwad

Signals and Systems:- Signals and Systems by Tarun Kumar Rawat.

Communication:- Communication systems: Analog and Digital by Sanjay Sharma.

Measurements:- A Course in Electrical And Electronic measurements And Instrumentation by A.K. Sawhney, Puneet Sawhney.

Control Systems:- Linear Control systems with MATLAB Applications by B.S. Manke.

Engineering Mathematics:- Advanced Engineering Mathematics – HK Dass. OR Higher Engineering Mathematics – B.S. Grewal.

General aptitude:- Buy any coaching study material for this. OR Quantitative aptitude by RS Agarwal.