can I use OSM to show the gps-coordinates of a way point for my gps ??
Not sure what you want to do.
Define a new waypoint on your GPS unit? You can find lat/lon of point on the map (on the openstreetmap.org homepage). Double-click to zoom and re-centre at the desired point, then click ‘permalink’ and read off the lat/lon coordinates from the URL in the address bar.
Or do you want to work with a waypoints file from your GPS unit? You can load a waypoints file into JOSM, then download the surrounding data to get a feel for where the waypoints are.