Open an image or begin a new document.

Activate Quick Mask mode using the left-bottom button in the image window. If a selection is present the mask is initialized with the content of the selection.

Choose any drawing tool. Paint on the Quick Mask with black to remove selected areas, and paint with white to add selected areas. Use gray colors to partially select areas.

You can also use selection tools and fill these selections with the Bucket Fill tool; this does not destroy the Quick Mask selections!

Toggle Quick Mask mode off using the left-bottom button in the image window: the selection will be displayed with marching ants.