Zxzax is correct that GIMP is an alternative to Photoshop, so the tool pallet is appropriate for editing photos. The typical workflow is: (1) stack layers, (2) mask layers, (4) composite. It’s rare to draw a circle on top of a photo. Usually it’s most efficient to select an area of a photo with a rectangle to isolate it on a layer, then use masking to refine the non-rectangular edge. If your goal is to draw a 2D illustration & work with gradient meshes, then Inkscape is an alternative to Illustrator. Both those programs are for illustrating in 2D and there are purpose-built features for drawing & editing shapes.
That said, GIMP does have real UX issues. For example, the controls and GUI elements don’t scale well to high resolution screens and could be much better on tablets like the Surface. They’re also missing CMYK color space support. But, I always assume they’re waiting for Adobe patents to expire to add those features.