hi,

see up to 2020 , students of final year engineering course were eligible to appear in gate exam, however in 2021 iit bombay has conducted gate exam, this year students of third year were also eligible to appear in gate. however from my opinion appear in gate exam in final year is more fruitful, you would get enough time to study and practice, however there is no limit for gate exam, you may appear in gate any time, after completing the b.tech also,  since there is no restriction on age limit.

but the main syllabus for gate covered in upto third year syllabus of b.tech, so in case you want to experience gate exam, you may surely apply while studying in third year.

hope you understand.