PoC of what I wanted:
fn init_root() -> Self::Root {
let win = gtk::Window::builder().title("uChoose").build();
let settings = win.settings();
let curr_dpi: i32 = settings.gtk_xft_dpi();
println!("gtk-xft-dpi: {:#?} ({}*1024)", curr_dpi, curr_dpi / 1024);
settings.set_gtk_xft_dpi(2 * curr_dpi);
win
}