One of the standard Gimp templates is US Letter 8.5" x 11" @ 300 ppi - suitable resolution for quality printing.
Divide into a 4 x 5 grid and each section is 637 x 660 pixels. Gimp works in pixels not inches.
One way is using the gmic plugin from http://www.gmic.eu In the arrays section 'montage'
If you make an image 637 x 660 pix and assemble new layers with your pasted in content up to the 20 layers
Then apply gmic montage with suitable settings you get rows and columns that print on US Letter size paper. Of course you might want to make a little less than 637x660 and finally resize the canvas to give a small margin around - a matter of choice.
Divide into a 4 x 5 grid and each section is 637 x 660 pixels. Gimp works in pixels not inches.
One way is using the gmic plugin from http://www.gmic.eu In the arrays section 'montage'
If you make an image 637 x 660 pix and assemble new layers with your pasted in content up to the 20 layers
Then apply gmic montage with suitable settings you get rows and columns that print on US Letter size paper. Of course you might want to make a little less than 637x660 and finally resize the canvas to give a small margin around - a matter of choice.