Yes, however it depends. If you apply those to your picture in gimp on one or two new layers, and save the image as a jpg or png (which compresses all the layers into one layer) then you won't be able to undo; you have to save it in gimp format (xcf) which won't work with Maptool.

You could save 2 versions of your map - one withe the grid and one without. Import gridded version into maptool; adjust the maptool grid to fit and add your vision-blocking layers, then import the clean map, align over the gridded one, and delete the gridded one; Voila!