Hi Aspirant 

GATE exam is conducted in various disciplines.Most of the students prefer the subjects which they have studied in their B.Tech or any subject whose syllabus is close to their stream .

If you have done B.Tech in food technology your first choice could be GATE in Food technology.

Your code would be XE/XL.

You can refer the official website to know about the streams and their syllabus.Visit the link given below:

http://gate.iitm.ac.in/Syllabus

hope this was helpful.

All the best.