What you need is to create a new layer, trace by hand over the original on the new layer, maybe in red or something like that to distinguish the two, and then delete the old layer. The exact details about which menus and buttons to push for creating or selecting which layer your on I will leave to the Gimpers. I think Devin probably has the easiest solution given the type of map your trying to extract - unless this is a pretty simple part of a very extensive or complex set of them.