I looked at GIMP 2.10, but it's still hopeless in my opinion. To add an alpha channel to an RGB, you need to add it as a
Layer, which is just a horribly illogical way to think about it (again IMO). And then to add a greyscale image to the alpha channel, well there might be a way, but it was beyond me.
Luckily I found a free plug-in called SuperPNG for Photoshop CS3 (and up). It allows 32-bit PNG files with a proper alpha channel (not a Layer) and fully editable in the way that I'm accustomed to. Installation and use was not well-described in its PDF manual, but reference to a blog helped me work those things out.
SuperPNG
fnord software blog
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