Gimp is the best for linux users, of course that gimp have a different learning curve but if you have experience with photoshop probably will be easy to learn about GIMP, in youtube have a lot videos explaning the uses of GIMP
I'd say gimp is a very good tool, but a lot of functions aren't very user-friendly. In my opinion Photoshop is a lot easier to use. And you can download the old Photoshop Version CS2 and use it for free.
GIMP is good, but it is not as widely used and less powerful the photoshop. You can also try Affinity which is cheaper than photoshop, but also very intuitive to use, and is an up and coming tool. Best of luck on your GFX journey.