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This isn't a gamut problem, it's a bad coding problem.

"Professional graphics software" (i.e. Photoshop) absolutely does the wrong thing for color blending with the default settings. It's not a problem of being limited to the sRGB color space, it's about the math in use being outright wrong for use on that color space. You need to go into the settings and tick some checkbox to have some hope of it doing the right thing ("Blend RGB colors using gamma"), and even then I bet it only applies to some operations like blending itself, not scaling and other filters.

This is because all non-linear image encoding modes, like sRGB or straight gamma, should be viewed as compression. You don't perform mathematical operations on compressed data. It's like trying to mix together two MP3 files by literally taking the bytes and averaging them out. That's madness and does not work. You need to go into a linear format, like raw PCM, to be able to do that (with the right word size). Same with images: you need to convert from sRGB to linear light to be able to do math on them. But 95% of software, including "pro" software, doesn't do this, it just blindly does math on compressed (gamma encoded) data, and the result is ugly (we're just used to it).

Of couse, with Photoshop you can set your image mode to linear light and then things work fine... but that's not because linear light is required to make this stuff work, it's because Photoshop is broken when not using linear light and doesn't compensate for the encoding.

Meanwhile GIMP, a few versions ago, fixed this whole mess and actually applies blending and filtering in linear light (regardless of what mode your image is using in memory), making its blending and filtering much more pleasant and correct than Photoshop's. So yeah, it may not support CMYK or other "pro" features... but for a good chunk of basic editing in RGB space, you're actually going to get more correct and visually appealing results if you use GIMP than Photoshop.

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