The earlier you begin, the more time you have in your hand. You must decide which paper to appear for (related to your B.Tech of course). Go through the present GATE syllabus for that paper. Chances of drastic changes in syllabus is quite small in an exam of this scale. Start studying key concepts from your engineering curriculum and at the same time, consult GATE specific books so that you get better at solving multiple choice questions. Once you have covered some topics, start solving those topics' questions from last years GATE question papers. When you are in 3rd year or towards its end, start taking mock tests. Good Luck!!!