You'll probably be sipping on a cup of tea, cowering in the corner or taking a peaceful afternoon siesta as far as I know of what GATE Aspirants do after the exam.
Jokes aside though, I think it's a really good thing that you're inspired to attempt GATE. Putting in a lot of effort right now is actually a good thing to do, as as you get closer to the exams you won't need to strain as much. I'd say 8-9 hours is a bit too much at a stretch but if you can segment them into 1-2 hours throughout the day it would be ideal. Also, remember to get an adequate amount of sleep and always eat on time (This is very important).
I personally know a lot of people who have switched from B.E or B.Tech ECE to CSE for their Master's, it's not impossible but you'll have to put in a good amount of effort as you'll be diving into a different world compared to what you've been learning these past 4 years. CSE is not tough, it's all logic, and honestly all you need is practice, lots of practice.
Also try to sign up or purchase a Mock Test series, it will really help you out especially in understanding the marking schema and also time management.
With that being said, I wish you all the best in your future endeavours.
