I created a brush following the advice of How to make the brush with the side of an even size (GIMP)? and from that tutorial https://www.gimp.org/tutorials/Custom_Brushes/.
I got what I wanted (in part).
When I make lines (click + shift + click) an error occurs.
Apparently the center of the brush is not set correctly, and the worst is that it varies depending on the entire image. Short lines do not change, the problem is evident when I make longer lines.
The problem is that a 10x brush has no centre pixel, because 10 is an even number.
The solution is to create an 11 x 11 px brush, but with a 10 x 10px black square inside it. This will give the brush a centre pixel, thus eliminating the problem.
Example Brush, greyscale, saved as .gbr
Next choose the Pencil tool, select the brush you made, set the brush size to 11px (because it is actually 11px, although the black area is only 10px)
In preferences set the default grid to 10px, enable Show Grid, enable Snap to Grid.
Draw with the brush as normal, the problem is gone. Don't zoom in too far, since the snapping doesn't work so well zoomed in.
If the brush has an even number of pixels, there is no pixel at the center... Using the paintbrush instead of the pencil does result in the brush straddling the track evenly.
