Yes, if you devote your maximum time in your preparation and if your concepts are clear and strong then 3 months are enough to clear Gate exam. Apart from this, you can follow below given preparation tips as well -

1. Know the syllabus very well as this is the first step and you will have to understand what you are up against and this information you need before you plan and then begin the actual preparation.
 
2. Read standard books only as there are many books available in the market so it is advisable to use the books which make all your concepts clear and which are easy to understand.
 
3. Understand the weightage of topics properly and accordingly devote time because I wont suggest you to give more time to those topics which has less weightage but those topics should not be ignored.
 
4. Proper Revision plan should be followed as this would be helping you alot.
 
5. Mock Tests and Practice Tests should be done as this will help you to learn time management skills and this will also make you familiar with the exam pattern. Also, to make the best use of mock tests, better to check test analysis report properly as this will show where all you are still lacking or where all you have to work really hard.

Hope this helps!!!