The best method to prepare for exam is to do prep along with semester. Give stress on problem solving because gate is fully problem based exam. Solve as many problems as you can.As far as books are concern the best books are that you have followed for your semester exam. For example for Networks you have read Hayat and Kimmerley in your semester, than for GATE also read that book beacuse by reading that book you will remind all concept.For problem solving best book is GATE EC By RK Kanodia. this book includes excellent problems with solution.