OP
My advice to you would be the same advice that I would give to anyone who is concerned about a loved one getting involved with a cult.
The first rule is not to argue with or oppose your DD’s position. Your DD will have conflicting / ambivalent feelings about her position and if you become entrenched in one position it will leave her occupying the opposing position. This is especially true of older children and teens as conflict with parents is a natural part of growing up and kids finding their own way in life.
The second rule is to be honest about not knowing everything and to not try to convince your DD of anything. It is important to genuinely come from a place of not knowing and wanting to understand. Let you DD know that you are open to her educating you. Ask her opinion about this and other issues. Resist any impulse to argue.
The third rule is to stimulate your DD’s natural curiosity and to ask her to explain things about the gender issue that you do not understand. It is extremely important that your DD feels empowered by you and that she is the one doing the teaching and educating and that she is the one in control. Other readers here have posted suggestions about gender critical organisations to contact and I would suggest that you follow these up. Find some issues about safeguarding, the links between cults and TRA activists, whatever is suitable for your DD’s age, and ask her to educate you about the issue. You can honestly claim to be confused as the whole thing is complex and confusing. Do not argue, simply do your best to inspire an enthusiasm for research on the part of your DD. Be extremely careful not to oppose your DD or argue.
The whole point should be to encourage your DD to research things for herself and to retain the capacity to think. The capacity to retain and use cognitive faculties can be undermined by the various discourses promoted by the TRAs and it is essential to keep your DD on side and thinking.
I would even suggest arguing some of the TRA positions as a demonstration that you are engaging with the debate, e.g. “could it be right that being a man or a woman is simply a matter of what you think to be true rather than what your body indicates?” and “maybe Steffoknee really is a 6 year old girl?” “people say that this JY person is a bully but others say that is just transphobia - what do you think?” etc.
The way that cults operate is that they provoke and instigate conflict between their followers and the families of their followers. They are skilled at creating and environment in which familial alienation occurs, at which point they separate their victims from their families and exploit them. The worst thing that a family can do is to feed into this by entering into arguments and conflict with their loved one.
Stay supportive, stay thinking and keep communications channels open.
Just my 2p worth
All the best