If you have the letters as a text layer, the cleanest result (and the only one that can produce sharp corners) is:
Layer > Text to path-
Edit > Stroke path(in “Line” mode, and use a reasonable miter limit, such as 3, if you use miter joins), on a new layer, above or under the text layer.
There is also my ofn-outline-layer script for the general case of something on a transparent background.