Gimp is a raster (bitmap) graphics tool. Works with (usually) square pixels. The semi-transparent ones you see are 'anti-aliasing' pixels added to give the impression of smoothness.
If you want a solid line of single pixels then dis-able the Colour Select Tool anti-aliasing and increase the threshold value to 50- 70 something like this. Then fill the selection on a new layer.
However Gimp is not the tool for vector work. Inkscape is and your original png converted to a SVG looks like this.
Edit: If a black / white image is ok for Qgis then you can use The threshold tool Colors -> Threshold
If you want a solid line of single pixels then dis-able the Colour Select Tool anti-aliasing and increase the threshold value to 50- 70 something like this. Then fill the selection on a new layer.
However Gimp is not the tool for vector work. Inkscape is and your original png converted to a SVG looks like this.
Edit: If a black / white image is ok for Qgis then you can use The threshold tool Colors -> Threshold