Not really something any sane person would use GIMP for, but multi-layer GeoTIFF files are fairly common. GeoTIFF is just a TIFF file where the tags are used to

Not really something any sane person would use GIMP for, but multi-layer GeoTIFF files are fairly common. GeoTIFF is just a TIFF file where the tags are used to store geospatial reference data. So you can have one layer contain elevation data for an area (TIFF supports floats as pixel values), one layer with aerial photography of that area, another layer with aerial photography from a thermal camera, etc.