I have a polygon, imported from a shapefile in QGIS. I would like transform it to a linestring instead so I can save it as a GPX file later.
How can I transform a polygon to a linestring in QGIS?
Assuming you're using QGIS >= 1.6.0, the menu option you need is Vector|Geometry Tools|Polygons to Lines, which will create a new shapefile with all the attributes of the original.
In QGIS 2
Vector > Geometry Tools > Polygons to lines
In QGIS 3
Processing Toolbox > Vector Geometry > Polygons to lines