I cannot tell you how to do them all at once in gimp - only by using the plugin G'MIC and then using montage -custom - scale to make your desired X x Y.
In gimp you can use grids or guides. Looks like you have some there. Try snapping. You need to go to preferences to make this a default and you might want the snapping distance to be more than one.
[attachment=7820]
In gimp you can use grids or guides. Looks like you have some there. Try snapping. You need to go to preferences to make this a default and you might want the snapping distance to be more than one.
[attachment=7820]
