You can place text in Gimp but I have found that if you want to place any text on a path (in other words on a curve of any kind) you would be better to learn how to use Inkscape. Gimp is supposed to be able to do it but it sure seems to get gobbledy gooped up.

Basically, what I do is save a full sized version of my map as a .jpg, open it in inkscape or import it, create a new layer for the labels and go to town. Then turn off the base layer and export just the text layer as a .png which I then pull back into gimp as a new layer in my original .xcf

Hope that helps. I was lost when I first tried this as well until RobA helped me out. I think he may have a mini tutorial or something on this now that I think about it.