Generally, the minimum time period of 6 months is enough to start with GATE preparation. Also it depends upon your grasping ability and how much time you can invest into it sincerely. As the syllabus is vast and so if its your first attempt you might panic at start.

So plan accordingly. Also I will suggest you some basic yet effective tips to follow.

First, be clear with the syllabus.

Second, be clear with the paper pattern, marking scheme, important sections and topics too.

Make a systematic time table and give attention to the difficult sections before planning to leave the topic.

Make notes, definitely it shall help you in memorisation.

Make a notebook separetely for formulas and fundamentals. Do not mix it with the theory. Above of all make sure you do appear for tests and mocks to anyalse your preparation.

Good Luck.