For going through the GATE examination it's always better that you prefer many books rather than a single book.You can prefer the core subjects books of standard authors like:-
Electronic Devices and Circuits:-William H. Hayt
Pulse Devices and Circuits:- J.Milman and H.Taub McGrawHill
Power Electronics:- J.Milman and H.Taub McGrawHill
Linear and Digital Integrated Circuits:-Ramakanth.A.Gayakwad
Signals and Systems:-Oppenheim Willsky
Communications Systems :-Simon Haykin
Digital Communications:- John Proakis
Engineering Electro magnetics :- William H. Hayt, John A. Buck
Digital Design Principles:- John F. Wakerly
Since the above mentioned are your subjects right from the first year hence its better that you follow the above mentioned standard books.
For maths books you can prefer Pearson Educations books i.e
Engineering Mathematics:-Anthony Croft, Martin Hargreaves, Robert Davison.
LINK FOR DIGITAL COMMUNICATIONS NOTES: