Cannot convert from rgb to monochrome

[Ofnuts]

When I create a 15000x15000 image, Gimp tells me the image is 2.1GB (265MPx, 4 bytes/pixels, likely the existing layer, the rendering mayer, a selection mask... This is dangerously close to your max virtual memory limit (around 3.5GB in 32 bits, 4GB theoretical minus some address space for the system needs...). However, if the Gimp tile cache size (Edit>Preferences>Environment>Tile cache size) is set to something reasonable (2GB or so) Gimp will swap to disk without using that much memory (but that will be slow).