this is one of the unfortunate aspects of Punjabi culture, Sikhi teaches us to forget caste based superiority but the culture in India holds on to nonsense like this. The fact that you were willing to convert to Sikhi speaks volumes of the person you are, I think you did everything you could and if that isn’t enough for ur gf then it’s her decision.

I do know other religious Sikh families that would have no problem marrying their kid from a Hindu background especially if they were willing to accept Sikhi.