Sometimes this effect is a called a shadowmat, although probably not in the example you posted.
This kind if typography is mostly done in a vectorgraphics program with offset path, but you can do it with a trick in Gimp too:
To get sharp corners you stroke the path with white, while having the Quickmask active.
Im describing the technique here:
http://gimp-science-labs.blogspot.de/201...al-ii.html
I think it was RobA who came up with it in a thread on gimpchat, here:
http://gimpchat.com/viewtopic.php?f=8&t=6973
The Quickmask technique steps are:
01. Have your text ready on one layer and convert it into a path.
02. Activate the Quickmask (Shift + Q) and make sure white is your FG colour.
03. In the paths tab, stroke the textpath with a thick width.
04. Deactivate the Quickmask - a selection will appear.
05. Convert this Selection into a Path.
06. This might need some clean up with the Paths Tool.