Important topics for GATE are as follows-

Electrical Engineering-

Engineering Mathematics

General Aptitude

Network Theory

Control system

Power system

Electrical Machine

Power Electronics

Signal and systems

Mechanical Engineering-

Engineering Mathematics

General Aptitude

Manufacturing science

Industrial engineering

Thermal Science

Fluid Mechanics

Strength of Materials

Computer Science-

Engineering Mathematics

General Aptitude

Theory of Computation

Computer Networks

Data Structures and Algorithms

Electronics & Communications- 

Engineering Mathematics

General Aptitude

Network

Signals and Systems

Control Systems

Analog Circuits

Electromagnetics

Communication System

You can also use the below tool to know about the important topics with their weightage and you can also check your preparation level.

Click on the below link-

www.engineering.careers360.com/prepmeter/gate-cse?icn=gate-cp&ici=forum