A couple of months ago I upgraded GIMP to version 2.10.20 and was very impressed. I wouldn't hesitate to use it for skinning. That said, I use it for my graphics work in AH3 objects and the terrain editor.
The 2.10.20 version
includes built-in Normal mapping, unlike the older versions where you had to install a plug-in. I don't think either of these powerful programs are easy to learn.
Of less interest to skinners, it handles 32,000 x 32,000 pixels x 32bit bitmaps if you have the hardware and memory (64GBs DDR4) to handle it. If not, while using such gigantic image layers, it's like the 1980s; grab lunch while you wait for the last operation to finish.
If you're using GIMP, it's well worth the time upgrading. I had a few issues with losing my preferences. Pretty common with any upgrade these days, but I had copies on another computer making it easy, if time consuming, to see what changes I'd made.
I recommend making screen shots of your important preferences, if you've done much customizing.