I've done exactly that tonight. I prefer the JOSM editor to Potlach for major work, but the simple thing is:
Open JOSM with the GPS file. It nicely colour codes them by speed so if I stop to go over a stile, I don't need to record the place, just look for the red dots.
Download existing route data from OSM.
Draw the roads on, tagging using the presets.
Click upload.
New tracks appear very soon on the site.