You should have a solid understanding of concepts in order to work with Analog Electronics or networks. There are many books on Analog electronics. Microelectronics circuits by Sedra & Smith is my personal favorite. It covers many important concepts, including transistors, transistors, Mosfets, and OPamps. It also allows you to solve exercises problems, which will help you understand better. The sollution manual is also available for download.
This book will be accompanied by Analog electronics and Network analysis GATE books by Kanodia. GATE books by Kanodia are excellent for communication and electronics. The book provides a brief explanation of each topic as well as many problems. Many questions are asked about transistors, mosfets, and OPamps within GATE. Solving more problems will help you understand the concepts better. You must apply the concepts you have learned, which is necessary for the GATE.
So these two books would be sufficient for GATE.