Have you done your objects with the path tool? That would be the correct way when doing diagrams/floor plans etc.
If so, you can easily merge all the paths (objects) on same layer, and stroke them with a given width (thickness).
If you had asked me how to do this from the beginning, my answer briefly would be:
1. Export the openstreet map as SVG. You can download a SVG version by using the share button.
2. Edit it to your heart's content. I couldn't open the SVG in Gimp (huge input lookup error), but it can be opened in Inkscape and probably fixed with scaling and optimizing the paths so that it opens in Gimp, have to try later.