Since you're bringing a vector image into a raster based program then any editing you do to the image in the raster program is going to give it that pixelation. especially if you enlarge it. If you're not resizing it then it may be due to the compression format you're saving it in typically .jpg or .png. PNG would be preferred since it's a "lossless" format meaning it is reconstructed exactly as it was before.
I don't use inkscape or gimp, but its pretty much the same as going from illustrator to photoshop. Check you're settings and see what you can change.
another idea would be to use the marquee tool in gimp and then fill it with black since its basically two rectangles put together.
Hope this helps. good luck