GIMP is still free. I made the switch from Photoshop to GIMP years ago. Never missed it for photo editing, creating logo's and other images, or designing large prints.
I've used Photoshop forever, mostly for image manipulation instead of full on graphic production. I've found the web editor Photopea[0] to scratch most of the itch these days.
I've been trying to use GIMP for years (since Corel destroyed Pain Shop Pro). I don't do a lot of photo manipulation so I don't put in a lot of time learning. PSP had a UI that was discoverable for an amateur occasional user. GIMP has a UI that is completely inscrutable. +1 for Photopea, it has become my go-to.
GIMP doesn't do 1% of what this does even if you don't have a problem with GIMP's UX. It's not comparable.
This. GIMP is a fine casual image processing tool. Its great in what it does. But you can't begin to compare it with Affinity or Photoshop.
I am not familiar with either, but am curious to know about their differences and intended workflows. Could you elaborate on your comment if time permits.
What can I not do with GIMP that I can do with the other software?
You can eventually do anything they can do with GIMP. It just takes a lot more time and effort and the results are less flexible. I say this as someone who tried to do it all in GIMP for many years before moving on to Affinity's software. It's the wrong tool for most of it because it's a raster graphics editor. No amount of better UX could address that. It doesn't have the tools.