I was like you when I wrote GATE in 2011. My sole aim ws to get a better package and there is nothing wrong with it. Fortunately, I got into IISc. Getting into IISc or IIT will change the whole perspecive of life. It willl make you realise what is important and what is not and you should never run towards packages. Not to mention that in these top institutes you will get to learn a lot of new things and study with the most bright students around the country, The atmosphere of competition here will be very healthy, not like most of the Btech colleges where every one else will try to oull you down.
So go for it. Write GATE again. Not to increase the salary but to experience something entirely different. I will suggest join a good coaching institute (weekend batches will suffice). I year is more than enough time to prepare for it.
On the matter of age, dude the time will pass no matter what you do. Why not use it to achieve something what you want? You will have your whole life to work.
All the best.