Unfortunately, Gimp is not suitable for working with 9x% of graphics professionals. It will not open many PSD files at all, and gets the colors and layers info wrong on some that it will open.
The closest thing to a 'solution', of which I am aware, is to use a Virtual Box running Windows. Even an old XP-compatible (2006) version of Photoshop will open everything that has been thrown at me. Gimp hasn't caught up to that year in the PSD department, sadly.
Please someone tell me I am wrong, as I waste a lot of time with this setup.
Further digging found this:
psd to png - software or website that converts all the components?
... indicating you can use imagemagick's 'convert' command to pull out all the layers separately. This is better than nothing, though if you need to capture a select-set of layers, together, in context, YMMV.