Seattle decided from the very beginning that Gellert was her big brother.
So...
Seattle: You're special. Starting today, you're my exclusive playmate.
Gellert: ...
Seattle: Big bro, listen. Love should start when you're young. Albus would make a wonderful wife. Go for it, bro! The family supports you!
Gellert: ...
Seattle: Big bro! I woke up here with Anna, and it's been more than twenty years. Honestly, bro, you've gotten ugly.
Gellert: ...
Seattle: Big bro! This boy is so cute. He's mine!
Newt: [shy]
Gellert: ...
And then...
Gellert: What made Albus Dumbledore like you so much?
Gellert: You think saving a little girl with an Obscurus means you won't go to prison?
Gellert: Death sentence! To be carried out immediately!
Gellert: Mr. Scamander, do you think Dumbledore will mourn you?
Newt: ... No, I don't think so.
The story is lighthearted. The female lead is a complete looks-obsessed girl. No transmigration, no rebirth, and a slight gift for prophecy.
Gellert really does think that cute little Newt first won so much favor from Dumbledore, then ran off with his little sister, so he's getting the gas stove treatment.
Ariana won't die. Why? It's all because of a Time-Turner. Let me recommend my other two works as well. Feel free to check them out if you're interested.
Theme: How could I possibly tolerate GGAD breaking up?