GIMP 3.0 includes a new experimental selection tool "Paint Select." This tool allows you to use a brush to progressively select an object or area in an image. While it is a very nice feature, it still has some stability issues. Therefore, it is not visible by default. You will need to enable it in the Playground section of the Preferences Dialog to test it out. You can access this section by running GIMP 3.0 with the --show-playground flag.
https://www.gimp.org/release-notes/gimp-3.0.html#experimental-tool