hi,

see there is no limit for applying in gate exam and if a student has already completed b. tech course or are in final year are eligible to apply for gate exam, and there is restriction of choosing any subject, like btech in mechanical student opt for gate exam from computer science stream, now see, there is no particular subject like dairy technology is in gate syllabus, however I think if you self study you may opt for biotechnology or any other topic you may find similarity.

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