Clear

https://docs.gimp.org/2.10/en/gimp-edit-clear.html

The Clear command deletes everything in the current selection.

or by using the keyboard shortcut Delete. On macOS, it’s fn+delete http://rat32.com/rat32/2013/08/22/solution-to-no-delete-key-in-mac-for-deleting-objects-in-gimp/