If you want to do it, then do it. If you enjoy the sport then keep doing it. Don't let other girls scare you away from the sport. You're not their to be best friends, you're there to learn gymnastics. It doesn't matter if they like you or not. You can take a tumbling class where all they teach is tumbling. You don't do bars, beam or vault. Just tumbling. You could also take private lessons with a coach. They can help you individually, some people just need that one on one attention. And you've only been doing gymnastics for a few months! Don't be so hard on yourself. Gymnastics is not something you can learn in just a few months.
If your not enjoying it then think it though properly but it doesn't mean you have to give up gymnastics completely. Try and find another local club that you will enjoy. Although if you don't want to stop because you feel you're benefitting from it then take no notice of the other girls. If you want to carry on then carry. Don't let them stop you from going. Also, if you do want to focus on tumbling then maybe tumbling classes would be good like other people said above. Just remember, do what makes you happy.
Tumbling is just 1 thing out of gymnastics. If you want to get better with tumbling take just a tumbling class that is at your level. You will learn more and faster because you are only focusing on tumbling and not the rest of the events in gymnastics. I do encourage you not to quit unless you are going to just a specific class like tumbling. Gymnastics keeps you very fit and flexible and will help you in the long run with many things; but if you are not happy with what you are doing then I think you should quit.
if you want to get good at tumbling, go take tumbling classes, it will concentrate more on tumbling than just the whole of gymnastics. of course
If you want to quit/make a change, DO IT !