Someone has guided you the wrong way. Gateforum book is not a good study material. I mean no one can really study anything from it. You can practice a lot of questions and for most exams where questions are repeated you can do well- but not in GATE. If you have studied all subjects you can use the Gateforum book for testing your problem solving skills- again never believe the solution given. Many of them might be wrong.
And for practice problems, previous year GATE papers are a much better resource. Again you should develop your own methods to solve the questions and understand the concept behind each question. Because without this you cannot get into an IIT.
This might be useful in forming a strategy: http://gatecse.in/wiki/Category:GATE_Experiences