While GIMP is my favorite general purpose image editor, it lacks a few very important features. In example there are no simple and easy tools to create and manipulate shapes, lacks any non destructive diting features and layers. But they are working on it and 1 or 2 months a big update is coming, with some major improvements in non destructive layer features.

Krita is also an excellent tool. While it has a focus and marketing on painting, it is still good enough to edit any image similar to GIMP. Even the popular effects addon GMIC can be used. There are bunch of non destructive effect layers, directly integrated vectors with enough shape tools. But on the other hand the text tool is abysmal, compared to GIMP.

It highly depends on what you expect from a Photoshop alternative and what you want do.