Use ellipse select tool with a fixed aspect ratio of 1:1. Then Select -> Border and choose a width. Then fill it with a color.

Like yeah, it is 3 steps instead of having a dedicated “Shapes” tool with a bunch of options that would do the same. But GIMP is kind of built around selections, you do need to learn to use them if you ever want to use GIMP effectively. It’s about as important as understanding layers.