In the meantime I found a way:
1. Open in JOSM a very small window in a new layer that has only one segment of the relation in it.
2. select the way and select the membership and edit the membership
3. press download and cancel
Now you have all the ways in the layer and can save it as GPX.
But if you open this GPX again, it doesn't look nice :-(
The ways are not in the right order.
Now I need a program to order the tracks.
Discussion
Comment from wieland on 30 August 2008 at 18:54
In the meantime I found a way:
1. Open in JOSM a very small window in a new layer that has only one segment of the relation in it.
2. select the way and select the membership and edit the membership
3. press download and cancel
Now you have all the ways in the layer and can save it as GPX.
But if you open this GPX again, it doesn't look nice :-(
The ways are not in the right order.
Now I need a program to order the tracks.
Comment from emka on 10 November 2008 at 17:18
here is another way:
1. go to http://betaplace.emaitie.de/webapps.relation-analyzer/
2. enter relation ID, click analyze
3. open the link "Download as GPX file"
Comment from wieland on 10 November 2008 at 17:38
Another way is the api:
osm.org/api/0.5/relation/id/full
Than convert osm to gpx e.g. with gpsbabel.
But thanks for the hint.