Gimp certainly has its GUI problems - coming back to it after using Darktable has been quite painful. I can accept it has GUI issues. However, for what it does, the only tool that is better is Photoshop. Sure, Lightroom/Darktable has a lot of nifty convenient stuff with a better GUI, but they're simply not a viable replacement for Gimp.

There are other photo editing tools with better UI, but poorer features, sadly. It would take a ton of work to catch up in terms of features.