GF's friends DISLIKE me. How to be better than her EX and show them I'm actually great?

Sadly you can't make some people change their minds on others or else the world would be much easier. However, I would have a word with your girlfriend though and talk to her about these horrible comments being made at you and see whether she could have a word with them. It's unacceptable for her to make that OK and if she doesn't then she clearly doesn't care because she should defend you.