I started using Photoshop at version 3.0. and used it for several versions until it got too expensive. I moved to GIMP for several years, and got quite good using it, but then I had an opportunity to try Photoshop again. I hunkered down and learned the "new way" of using Photoshop, with layer masks, adjustment layers, etc. Gimp can do a lot of similar things, but not as easily, as quickly, or as accurately. (accurate may not be the right word, but it is much easier to fine tune things in Photoshop)
In short, once I learned Photoshop, I could do the same things that I could do in GIMP but better, and there were things I could do in Photoshop that were just too complicated in GIMP.
Different people may have different needs, but GIMP is not a Photoshop replacement