What should I do if my parents are against my girlfriend? They want me to marry a girl of their choice and not to marry my girlfriend. They gave me choice "live with us and leave your girlfriend or live with your girlfriend and leave us."
If you're over 18, you do whatever the heck you want. It's your choice. You either go get a job and live with your girlfriend. Or you listen to mom and dad until they find you a nice wife from Gujarati.
I am not all that familiar with the cultural practice, but .. If you have a choice .. Consider yourself fortunate to even HAVE a choice. This is something you will have to think about thoroughly. Choosing love over tradition for most is not a difficult choice in this country .. But with your up bringing, it WILL be far more challenging. That is something ONLY you can decide. Your decision will involve not just who to marry, and potential living conditions, but it will also involve breaking or staying with tradition. Rest assured there will be consequences and benefits involved in either direction you choose.
You haven't said, but I suspect you live in a culture where arranged marriages are normal. In fact, arranged marriages are much more successful than love matches. To get your own way, there is only one solution: Tell your parents you need more time to get more education in the meantime, help your girlfriend to get more education too. When both of you look good on paper in a couple of years, market her as a good prospect for a match. Use the language set aside for these arrangements. Actively involve her parents. But consider that a lot can change in a couple of years. I think you are not ready for marriage against the wishes of your family.