You'd be surprised how many people use non-GTK X11 Emacs. The reasons they give include being able to connect and disconnect from multiple X servers (GTK has longstanding unfixed bugs here) and being generally lighter on the system than GTK is.

"Pulling [GTK] out into its own frontend" means taking GTK support out of where it is now, Emacs' general-purpose X11 code (where GTK is one of many supported X11 toolkits) and putting it in a new top-level window-system that's a peer of NS (for OS X) and Windows support. The new GTK window system wouldn't be allowed to use X11 functions directly. Under this architecture, Emacs would be much closer to a well-behaved GTK program and could take advantage of cool GTK tricks like Broadway support.