PMC Editing Wiki: Tools GIMP, see also GIMP: High Resolution BMP Wont Open.
Large, HUGE, images!
20,480 x 20,480 resolution 9 layers XCF image is too much with 32gb RAM. You ran out of RAM because GIMP v2.10.6 doesn't respect the tile cache value, if you set it to default 16gb it will happily uses 27gb and your windows 7 runs out of RAM.
When working with 20,480 x 20,480 pixel resolution PNG images (ArmA 3 global mapper exported satellite texture tiles), clone brushing clouds off, then export / save the PNG, it will be extremely slow. This resolution image is very heavy for GIMP, lower resolution image is recommended.
Instead of using 4 x 4 tiles with 20,480 resolution images, use 8 x 8 tiles with 10,240 resolution images.