I'm an Evangelical Christian with very traditional views.
I also believe that every single human was created with intrinsic worth, loved by God and has free will to live their lives as they wish.
I have gay friends. We don't speak about the issue but they take me at face value, and I them. I also have straight friends who live with their boyfriends, friends who have children with their partners, Muslim friends, (many) atheist friends.
Our friendships are all based on kindness and how we treat one another. I, and everyone with 1 cm of nuance, can show kindness and care about someone and love someone without fully getting behind every single aspect of that person's life. One of my Christian friends has a trans sibling who she adores and has a good relationship with. My friend doesn't agree with the trans ideology but it hasn't separated her from her sibling. They leave that issue at the door.
Very devout Christians aren't all emotionless, hate-filled zombies who live in a cocoon. We can share Christ's love, which is for everyone, without having to endorse certain aspects of how a person lives their life. It certainly isn't all or nothing.
I really hope all goes well with sharing the news with the grandparents, should you decide to. Lying or hiding things will just create a web of deception and so it's best to bring things into the open, maybe with you bridging the gap for a while to see how things go.
All the best op.
Ps I've just noticed that this was posted under LGBQT children but it came up in my active, so I'm sorry if I'm stepping on anyone's toes.