Go through the syllabus very well and cover almost all the important topics in depth first.

In mathematics part, get great grip on calculus, vector, probability, binomial, and statistics. Everything is important in Network, Signals and System so do it properly. Revise the basic topics like carrier transport current, drift current, mobility and resistivity;  Poisson and continuity equations; P-N junction, Zener diode, etc. In electronic devices. Also go well with electric magnetism, control systems and communications.

Some of the good books that I can recommend you are RS AGGARWAL for general aptitude, Norma Nise for control system,  Van Valkenburg for network, Millman for electronic devices, and A Deaman for analog circuits.

Refer to the site below for more detailed information..

https://engineering.careers360.com/articles/how-to-prepare-for-gate-electronics-and-communication-engineering

Best wishes. Thank you.