GIMP lets you transform the selection of an image into a path. It also lets you transform paths into selections. For information about the selection and how it works, see the Selection section.
When you transform a selection into a path, the path closely follows the «marching ants». Now, the selection is a two-dimensional entity, but a path is a one-dimensional entity, so there is no way to transform the selection into a path without losing information. In fact, any information about partially selected areas (i.e., feathering) are lost when a selection is turned into a path. If the path is transformed back into a selection, the result is an all-or-none selection, similar to what is obtained by executing "Sharpen" from the Select menu.