The GUI is perfectly fine. I use it all the time and it does everything I need. Sometimes I have to look up how to do something but that's true of any program. What exactly is it missing?
This is what I sounded like when I was trying to convince my non-nerd friends that they should use Linux on their laptops. People have different requirements. If a modern interface that isn't blurry on modern screens is one of those requirements, then so be it. Convincing them that their requirement is wrong isn't going to fix GIMP