Audacity is another great program from experience, and the success of Blender speaks for itself.
Gimp, on the other hand, is a UX nightmare. There's a tutorial on how to draw a line, and you'll need it, because it requires a non-discoverable keyboard shortcut. Its feature set is either ages behind Photoshop, or there but non-discoverable due to bad UX, and the UX makes basic tasks excruciatingly painful.
Photoshop and IrfanView are the two tools I miss most from Windows, the latter even more than the former. Trivial tasks like "crop picture in clipboard and put it back on the clipboard/into a file" are pure pain on Linux, because the only thing that at least works reliably is Gimp with its horrible UX. One of the "lighter" (Paint.NET equivalent) tools left parts of its UI in my saved image. Others don't support the clipboard. IrfanView with Wine just isn't the same either.