How to prepare for gate exam by a below averge student

Solve last year question papers
Get expert in mostly asked topics
Try to focus only on studies
Read standard books.

By below average , I mean you are not fundamentally strong in your subjects . To clear GATE , one of the most basic requirement is that a candidate should be fundamentally strong with their concepts in subjects. To start preparation for gate you can  start by preparing individually per subject  . Assuming you give  6 months time for gate preparation , then prepare subjects for first 4 months , test series in 5th month and final revision in 6th month . To prepare for each subject , study standard textbooks , make notes and revise . For test series take any coaching institute test series and for revision take Made easy handbook