Yes, you are eligible for gate 2021 even if you are in 3rd year of b.tech.

Food processing engineering is about processing into food from raw materials. It is 4 year course. Even after completion of undergraduate course you can certainly find many jobs such as quality control executive., Production manager., Product development executive etc.,

If you want to have an master's degree you can go into courses such as

1. Master's of engineering in food technology

2. Master of technology in processing and food engineering

3. Master of technology in food and nutrition

4. Master of technology in food safety and standards etc.

You can even join the certification courses which will help you alot for your job and carrier.