Hmm, I think I didn’t express myself very clearly. I’m not being critical of OpenSUSE OR Gentoo. There are problems and issues with all operating systems, that might or might not affect any given “user+hardware” combination.
On OpenSUSE, darktable installed from OpenSUSE Tumbleweed worked perfectly with GIMP 2.9 compiled in a prefix. And I’m sure darktable compiled from source also would have worked perfectly.
On Gentoo, darktable compiled from source works perfectly. But using darktable from portage, there is a problem with the GIMP plug-in because of lua: currently Gentoo does supply lua 5.3, but looking at the Gentoo bug reports (and considering the lack of a lua use flag), darktable from Gentoo portage apparently hasn’t been updated to use Gentoo lua.