I use Libreoffice and GIMP a lot, also Scribus, which nicely integrates with GIMP, and Inkscape.
Nice to be able to work on the same files with the same apps across different platforms. My preferred OS is Linux, but I use Windows at the office, most of the competition are Windows or Mac only.
I actually like the multiwindow UI, great with multiple screens, I can work with the image on one screen and the layers and toolbox on another. But you can have a single window if you prefer.
Such a powerful image editor is never going to be simple, especially when some people try to use it as a drawing program, for which there are far better tools.