I'd be upset for sure initially. But it would make absolutely no difference in terms of loving them. I'd just want them to be happy and not in any type of abusive or hurtful relationship. There is too much shit in the world. It would be more difficult if they were a SJW or a religious zealot. But even then, I believe you have to try at least to work through it.
I'd pay for a significant life insurance policy on them in favor of myself and keep sending them backpacking to whatever shithole country currently has significant muslim terror activity. Maybe organize small parties for them in public places for celebrating them "coming out", by calling ahead. Get rid of the faulty offspring, earn sympathy upcummies and recoup monetary losses.
It'd be hard and difficult to accept the change, but I would try and support them, especially in the case of being trans if they have actual dysphoria. As long as they don't start participating in anything questionable and go through the necessarily counseling and steps to transition in the trans situation, and are closely supervised, I will support them. Just none of that weird tumblr ""nonbinary without dysphoria"" bullshit, not in my house, that's transtrending / "I just want to be special" and I won't stand for that. Very complicated subject. I'm bisexual so I at least understand the whole bisexuality/gay/lesbian angle myself, it's been the way I've felt for a long time. I just think you need to have dysphoria to be trans, anything other than FTM or MTF isn't going to fly, I consider myself supportive of the lgbt community but draw the line at "third gender" bullshit.
If my kid came out as LGB I wouldn't care, sure future kiddie I accept that you are gay/bi, but, here are the chores that you have to do today and tomorrow, don't forget to buy bread when you get out of class.
If it comes out as T or gender special, I would double the chores, you don't have time to think about gender identity when you are shopping the groceries, or going to grandma and grandpa's house to look after them. Also, I would put them in some extra curricular activity like learning other language or playing some sport. A chore a day will keep the gender delusions away.
As long as their safe and self-aware about their decisions I'll accept them
if they're Gay, Lesbian or Bi I'll be fine with that
with being Trans though, I'll ask or talk to them about why exactly they feel that way, if it's them experimenting with androgyny, having body image issues or whether they are experiencing some early signs of Gender Dysphoria. (so they don't end up "de-transitioning" as some have done)
They'd have to be in about their mid-teens though, any other actions taken about it will happen in their later adult years
I'll get them to a good psychologist so they can learn more about what's causing it and how they can manage it, if they think about going on testosterone or something, I'll tell them to be aware of the side effects those medications and procedures can do to them.
I would also warn them about being safe when going out late at night, to clubs or going on dates, considering the risks of drugs and other problems, and the fact that a lot of trans people get attacked or murdered
other than that my opinion on them won't change, I will still love them