GTK+-2 is obsolete now.

It would be good if we have pinentry with GTK4. But some features cannot be supported: grabbing input, asking window manager to center its window or to be placed on top of all windows. That's because GTK4 is a kind of pure toolkit not supporting backend control (of X11, Wayland, Windows, Mac, etc.).

Still, it is possible to provide a simple password entry window with GTK4, and there could be a way to enable fullscreen mode for that.