Most experts use commercial software like Autocad 3ds max over Blender for example, too. But what is your point? It's still expert software.
GIMP - Free and you only pay with your time and sanity.
Now if we could only come up with some word for people that exchange time for money...
Gimp is not expert software though. It’s an okay foss alternative to photoshop, but just because it has an awful UX does not suddenly make it ax expert software.
I think "alternative to photoshop" pushes it into the expert category. Non-expert would be something like MS Paint.
I don't consider Gimp to have radically worse UX either. I've got a couple of minor gripes but nothing major.