I mean Terminator is basically gTmux, right? They aren't quite the same: tmux is a terminal multiplexer, Terminator is a terminal emulator with tabs and panes. Both approaches have benefits and drawbacks.
If you want keyboard only, I don't think GUIs can beat TUIs. But yeah, there's a learning curve.
The problem with GUIs is a very display-session centric view. tmux/vim work fine over SSH, Terminator/gVim don't. With tmux, your sessions are separate from your terminal instance. If your X session crashes, depending on how you started tmux, you just have to relaunch a terminal and reconnect to tmux. This is pretty invaluable. So this separation is powerful, and IMO very Unix-y.