I am going to assume that you know how to use compositor nodes.
Is it just me or do the images look very similar? I can honestly barely tell the difference. If I had to put it into numbers I’d say I see a difference of about 2-4% in tones, with the GIMP image being 2-4% darker. I think it’s something to do with shading (shadows) itself, as the colours look brighter, not more saturated. Then again, I am looking at it from a f.lux-ed laptop with a screen loosely calibrated at 5.5kK.
If it’s not that it might be your display intent in colour spaces or the AO is not showing up in viewport. I’m not sure if AO shows up or not, but it would be worth a look; as I said it looks to me like the slide is casting a shadow on itself relative to the vicinity of its edges.
You will only be able to get to the other option through GIMP I guess, but basically what I think you will find is that the image’s display intent is set to perceptual. Try setting it to Relative colorimetric. It might change something, then again it might not because it generally doesn’t influece the shadows as such, just saturation and lightness.