In GIMP, a simple approach is to fix only that corner on a separate layer.

1. Duplicate the image layer.
2. On the duplicate, adjust contrast/tonality until the bright upper-right glare looks better.
3. Add a layer mask or erase selectively so only the corrected corner shows through.
4. Use a large, soft brush/eraser for smooth blending into the original image.

This works because you’re applying the correction only where needed, instead of changing the whole photo.

You can also try G’MIC (a popular GIMP plug-in), which includes many filters that can help with local contrast and selective enhancement. Depending on the image, one of its local adjustment tools may help tame the glare more naturally.

Keep expectations realistic: if the glare is strongly blown out, there may be little or no detail left to recover, so the goal is usually to reduce the distraction rather than fully restore the area.