See if you crack GATE in your 3rd year then also you have to wait to complete your graduation but by that time you can participate in the GATE Counselling and can reserve your seat in the required NITs/IITs or any other institution because third year students are also eligible for GATE. During third year after the declaration of your GATE result you can apply for different institutions through GATE counselling which is conducted by Common Offer Acceptance Portal (COAP) and Centralised Counselling for M.Tech/ M.Des (CCMT)  and if the seat is allotted to you and you accept the seat then also you will have to wait till your graduation gets completed but your seat will be reserved.

For complete details about GATE counselling kindly check out the link given below :-

https://engineering.careers360.com/articles/gate-counselling

If you are preparing for GATE 2022 then you should well aware of GATE 2022 syllabus so you can check it out from the link given below :-

https://engineering.careers360.com/articles/gate-syllabus