"On macOS, the main GUI experience is written in Swift using AppKit and SwiftUI. The tabs are native tabs, the splits are native UI components, multi-window works as you’d expect, etc. On Linux, the GUI experience is GTK using real GTK windows and other widgets.
Features such as error messages are not implemented with a specialized terminal view, we actually use real native UI components. The point is, while the terminal surface and core logic is cross-platform, the user interaction is all purpose-built for each operating system for a true native experience."
He seems to target GTK based on his statement:
https://mitchellh.com/writing/ghostty-and-useful-zig-patterns