Yeah I'm curious what exactly makes GIMP different. Photoshop and Pixelmator are bot pretty similar to use on a Mac. Is it just the layout of buttons? Or is there some "first principles" difference in the design of GIMP vs Photoshop? Is GIMP made for programmers and Photoshop for designers?

No, GIMP is for designers, too. I use Photoshop at work and GIMP at home. Both for very light work. Adding text to an image, putting a product into a mockup template, resizing, etc. Both programs are very similar - I don’t really notice much difference between them. And GIMP does some things better!

Like any program, there’s a learning curve. But there are lots of like-for-like features and layout choices between the two, so if you know one then the other is easy to adapt to.