I am final year in mechanical engineering
GATE is one of toughest entrance exam for engineers. It provides admission gateway for mtech, PSU.
Preparing for GATE should start in 3rd year and yes there is need to go through mock test.
Youth4works also provides mock test for gate you can refer those too. Even you can join some institutional online test series also.
While going through these test be well planned. Solve test to minimize your mistakes and improve accuracy.