>>4101298first you shouldn't be exporting as png because it's not an ideal format for photography. yes it's lossless, but you're not going to notice the difference between that and a high quality jpeg (11/12 or 12/12).
now for gimp. ensure you have color management enabled. then, before exporting, use Convert to Color Profile to convert the image to sRGB IEC61966-2.1. all images destined for internet consumption should be exported as sRGB.
