Krita supports 100000000 * 100000000 pixels, making it perfect for drawing long scroll murals. GIMP only supports 524288 pixels.
I am afraid that is not changing to this order of magnitude any time soon.
like,doubling, or even quadrupling the current amount, is a matter of changing a constant in the source code, and pray the user machine has enough memory (16X as much for quadruppling the linear pixel size on each of the image dimensions) .
For 100 X more, you really need the underlying engine to work with sparse images, keeping layers and tiles pixel information at different scales, so that selected tiles will be at full resolution, and then, use OS mechanisms to dynamically pull in image data into RAM when painting at zoom.
These are all nice to haves, and the GEGL foundations may allow these to be patched in in a consistent way in the future - but that future is not soon.
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At 300PPI (which is a lot for a mural, 150PPI would be fine) 524288 pixels is still an image which is 44 meters wideā¦
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40m, not 44, but either way, even 25 PPI is more than good for a mural this large.
(a single 8bit layer with no undo would take 256GB for an image in this maximum GIMP size - nowadays that there are these ludriculous machines to rent for the minute, it could be an interesting tour-de-force for GIMP one day 
)